Liquid crystal display device and electronic device

ABSTRACT

To provide a semiconductor device, a liquid crystal display device, and an electronic device which have a wide viewing angle and in which the number of manufacturing steps, the number of masks, and manufacturing cost are reduced compared with a conventional one. The liquid crystal display device includes a first electrode formed over an entire surface of one side of a substrate; a first insulating film formed over the first electrode; a thin film transistor formed over the first insulating film; a second insulating film formed over the thin film transistor; a second electrode formed over the second insulating film and having a plurality of openings; and a liquid crystal over the second electrode. The liquid crystal is controlled by an electric field between the first electrode and the second electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/628,459 filed Feb. 23, 2015, now allowed, which is a continuation ofU.S. application Ser. No. 14/322,999 filed Jul. 3, 2014, now U.S. Pat.No. 8,964,156, which is a continuation of U.S. application Ser. No.13/409,288, filed Mar. 1, 2012, now U.S. Pat. No. 8,780,307, which is acontinuation of U.S. application Ser. No. 12/977,217, filed Dec. 23,2010, now U.S. Pat. No. 8,130,354, which is a continuation of U.S.application Ser. No. 11/923,128, filed Oct. 24, 2007, now U.S. Pat. No.7,872,722, which claims the benefit of a foreign priority applicationfiled in Japan as Serial No. 2006-297009 on Oct. 31, 2006, all of whichare incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a semiconductor device and a liquidcrystal display device. In particular, the present invention relates toa semiconductor device and a liquid crystal display device in whichliquid crystal molecules are controlled by generating an electric fieldhaving a component parallel to a substrate.

2. Description of the Related Art

One of technical development strategies of a liquid crystal displaydevice is to widen a viewing angle. As a technique for realizing a wideviewing angle, a method is used in which a gray scale is controlled bygenerating an electric field parallel (i.e., in a lateral direction) toa substrate to move liquid crystal molecules in a plane parallel to thesubstrate.

Examples of such a method include an IPS (In-Plane Switching) mode andan FFS (Fringe-Field Switching) mode.

In an IPS mode liquid crystal display device, two comb-shaped electrodes(also referred to as comb-teeth electrodes or comb electrodes) areprovided over one of a pair of substrates. Liquid crystal molecules aremoved within a plane parallel to the substrate by a horizontal electricfield generated by potential difference between these electrodes (one ofthe comb-shaped electrodes is a pixel electrode and the other thereof isa common electrode).

In an FFS mode liquid crystal display device, a second electrode (e.g.,a pixel electrode in which voltage for each pixel is controlled) havingan opening is provided below a liquid crystal, and a first electrode(e.g., a common electrode in which common voltage is applied to allpixel) is provided below the opening in some cases. An electric field isapplied between the pixel electrode and the common electrode to controlthe liquid crystal. An electric field in a parallel direction is appliedto the liquid crystal, so that liquid crystal molecules can becontrolled using the electric field. That is, liquid crystal moleculesaligned parallel to a substrate (so-called homogeneous alignment) can becontrolled in a direction parallel to the substrate, so that a viewingangle is widened.

In conventional semiconductor device and liquid crystal display devicewhich control liquid crystal molecules, a pixel electrode or a commonelectrode has been formed of a light-transmitting conductive film, forexample, indium tin oxide (ITO) (see Patent Document 1: Japanese PatentNo. 3742836).

SUMMARY OF THE INVENTION

As described above, a pixel electrode or a common electrode has beenformed of a light-transmitting conductive film, for example, ITO. Inorder to manufacture a semiconductor device which controls transmissiveliquid crystal molecules and a transmissive liquid crystal displaydevice, a pixel electrode and a common electrode are needed to be formedof a light-transmitting conductive film. Conventionally, after formationof a light-transmitting conductive film, the light-transmittingconductive film has been processed to form a pixel electrode and acommon electrode by etching or the like. Thus, the number ofmanufacturing steps and the number of masks have been increased, andmanufacturing cost has been increased.

In view of the above, objects of the invention are to provide asemiconductor device, a liquid crystal display device, and an electronicdevice which have a wide viewing angle and in which the number ofmanufacturing steps and the number of masks are small and manufacturingcost is low.

In the invention, a light-transmitting conductive film is formed to useas one of a pixel electrode and a common electrode of a liquid crystaldisplay device without processing. Thus, the light-transmittingconductive film is not needed to be processed by etching or the like, sothat the number of manufacturing steps and the number of masks can bereduced and manufacturing cost can be suppressed.

Note that a liquid crystal element is acceptable as long as arrangementof liquid crystal molecules controlling the amount of light can berotated in a direction generally parallel to a substrate by a horizontalelectric field generated by potential difference between a pixelelectrode and a common electrode which is connected in common with aplurality of pixels in a pixel portion.

One aspect of the invention is a liquid crystal display device whichincludes a first electrode formed over an entire surface of one side ofa substrate; a first insulating film formed over the first electrode; athin film transistor formed over the first insulating film; a secondinsulating film formed over the thin film transistor; a second electrodeformed over the second insulating film and having a plurality ofopenings; and a liquid crystal over the second electrode. The liquidcrystal is controlled by an electric field between the first electrodeand the second electrode.

In the invention, the thin film transistor may be a top-gate thin filmtransistor.

In the invention, the thin film transistor may be a bottom-gate thinfilm transistor.

In the invention, the first electrode and the second electrode may belight-transmitting conductive films.

In the invention, one of the first electrode and the second electrodemay be a light-transmitting conductive film and the other thereof may bea reflective conductive film.

The invention also relates to an electronic device provided with aliquid crystal display device formed using the invention.

The following description is for structures applicable to a liquidcrystal display device and a semiconductor device of the invention. Thestructures described hereinafter can be applied to a liquid crystaldisplay device and a semiconductor device of the invention when needed.

Note that various types of switches can be used as a switch. Anelectrical switch, a mechanical switch, and the like are given asexamples. That is, any element can be used without being limited to aparticular type as long as it can control a current flow. For example, atransistor (e.g., a bipolar transistor or a MOS transistor), a diode(e.g., a PN diode, a PIN diode, a Schottky diode, a MIM (Metal InsulatorMetal) diode, a MIS (Metal Insulator Semiconductor) diode, or adiode-connected transistor), a thyristor, or the like can be used as aswitch. Further, a logic circuit combining such elements can be used asa switch.

Examples of a mechanical switch include a switch formed using a microelectro mechanical system (MEMS) technology, such as a digital micromirror device (DMD).

Such a switch includes an electrode which can be moved mechanically, andoperates by controlling connection or non-connection based on movementof the electrode.

In the case where a transistor is used as a switch, polarity (aconductivity type) of the transistor is not particularly limited sinceit operates just as a switch. However, when off-current is preferably tobe suppressed, a transistor of polarity with smaller off-current ispreferably used. As a transistor with smaller off-current, a transistorhaving an LDD region, a transistor having a multi-gate structure, andthe like are given as examples. Further, an n-channel transistor ispreferably used when the transistor operates with a potential of asource terminal closer to a potential of a low potential side powersupply (e.g., Vss, GND, or 0 V). On the other hand, a p-channeltransistor is preferably used when the transistor operates with apotential of a source terminal close to a potential of a high potentialside power supply (e.g., Vdd). This is because when the n-channeltransistor operates with the potential of the source terminal close to alow potential side power supply or the p-channel transistor operateswith the potential of the source terminal close to a high potential sidepower supply, an absolute value of a gate-source voltage can beincreased; thus, the transistor can more precisely operate as a switch.This is because reduction in output voltage does not occur often sincethe transistor does not often perform a source follower operation.

Note that a CMOS switch may also be employed by using both n-channel andp-channel transistors. A CMOS switch can easily function as a switchsince a current can flow when one of the n-channel transistor and thep-channel transistor is turned on. For example, a voltage can be outputas appropriate whether a voltage of an input signal to the switch ishigh or low. Further, since a voltage amplitude value of a signal forturning on/off a switch can be decreased, power consumption can bereduced.

Note that when a transistor is used as a switch, the switch includes aninput terminal (one of a source terminal and a drain terminal), anoutput terminal (the other of the source terminal and the drainterminal), and a terminal (a gate terminal) for controlling electricalconduction. On the other hand, when a diode is used as a switch, theswitch does not have a terminal for controlling electrical conduction insome cases. Therefore, when a diode is used as a switch, the number ofwirings for controlling terminals can be reduced compared with the casewhere a transistor is used as a switch.

Note that when it is explicitly described that A and B are connected,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included. Here, each of A and B is an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, or a layer). Accordingly, another element may beprovided in a connection relationship shown in drawings and texts,without being limited to a predetermined connection relationship, forexample, connection relationships shown in the drawings and the texts.

For example, when A and B are electrically connected, one or moreelements which enable electrical connection of A and B (e.g., a switch,a transistor, a capacitor, an inductor, a resistor, or a diode) may beprovided between A and B. In addition, when A and B are functionallyconnected, one or more circuits which enable functional connection of Aand B (e.g., a logic circuit such as an inverter, a NAND circuit, or aNOR circuit; a signal converter circuit such as a DA converter circuit,an AD converter circuit, or a gamma correction circuit; a potentiallevel converter circuit such as a power supply circuit (e.g., a voltagestep-up circuit or a voltage step-down circuit) or a level shiftercircuit for changing a potential level of a signal; a voltage source; acurrent source; a switching circuit; or an amplifier circuit which canincrease signal amplitude, the amount of current, or the like, such asan operational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit, a signal generation circuit; amemory circuit; or a control circuit may be provided between A and B.Alternatively, in the case where A and B are directly connected, A and Bmay be directly connected without interposing another element or anothercircuit therebetween.

When it is explicitly described that A and B are directly connected, thecase where A and B are directly connected (i.e., the case where A and Bare connected without interposing another element or another circuittherebetween) and the case where A and B are electrically connected(i.e., the case where A and B are connected by interposing anotherelement or another circuit therebetween) are included.

In addition, when it is explicitly described that A and B areelectrically connected, the case where A and B are electricallyconnected (i.e., the case where A and B are connected by interposinganother element or another circuit therebetween), the case where A and Bare functionally connected (i.e., the case where A and B arefunctionally connected by interposing another circuit therebetween), andthe case where A and B are directly connected (i.e., the case where Aand B are connected without interposing another element or anothercircuit therebetween) are included. That is, when it is explicitlydescribed that A and B are electrically connected, the description isthe same as the case where it is explicitly only described that A and Bare connected.

Note that a display element, a display device which is a deviceincluding a display element, a light-emitting element, and alight-emitting device which is a device including a light-emittingelement can employ various types and can include various elements. Forexample, as a display element, a display device, a light-emittingelement, and a light-emitting device, a display medium, contrast,luminance, reflectivity, transmittance, or the like of which is changedby electromagnetic action, such as an EL (electroluminescence) element(e.g., an EL element including both organic and inorganic materials, anorganic EL element, or an inorganic EL element), an electron emitter, aliquid crystal element, electronic ink, an electrophoretic element, agrating light valve (GLV), a plasma display panel (PDP), a digitalmicromirror device (DMD), a piezoelectric ceramic display, or a carbonnanotube can be used. Note that display devices using an EL elementinclude an EL display; display devices using an electron emitter includea field emission display (FED), an SED-type flat panel display (SED:Surface-conduction Electron-emitter Display), and the like; displaydevices using a liquid crystal element include a liquid crystal display(e.g., a transmissive liquid crystal display, a transflective liquidcrystal display, a reflective liquid crystal display, a direct-viewliquid crystal display, or a projection type liquid crystal display; anddisplay devices using electronic ink include electronic paper.

Note that an EL element is an element including an anode, a cathode, andan EL layer interposed between the anode and the cathode.

Examples of the EL layer include various types of EL layers, forexample, a layer utilizing light emission (fluorescence) from a singletexciton, a layer utilizing light emission (phosphorescence) from atriplet exciton, a layer utilizing light emission (fluorescence) from asinglet exciton and light emission (phosphorescence) from a tripletexciton, a layer formed of an organic material, a layer formed of aninorganic material, a layer formed of an organic material and aninorganic material, a layer including a high molecular material, a layerincluding a low molecular material, and a layer including a lowmolecular material and a high molecular material.

Note that the invention is not limited thereto, and various types of ELelements can be used.

Note that an electron emitter is an element in which electrons areextracted by high electric field concentration on a pointed cathode.

For example, the electron emitter may be any one of a Spindt type, acarbon nanotube (CNT) type, a metal-insulator-metal (MIM) type in whicha metal, an insulator, and a metal are stacked, ametal-insulator-semiconductor (MIS) type in which a metal, an insulator,and a semiconductor are stacked, a MOS type, a silicon type, a thin filmdiode type, a diamond type, a surface conduction emitter SCD type, athin film type in which a metal, an insulator, a semiconductor, and ametal are stacked, a HEED type, an EL type, a porous silicon type, asurface-conduction electron-emitter (SED) type, and the like. However,the invention is not limited thereto, and various elements can be usedas an electron emitter.

Note that a liquid crystal element is an element which controlstransmission or non-transmission of light by optical modulation actionof a liquid crystal and includes a pair of electrodes and a liquidcrystal.

Optical modulation action of a liquid crystal is controlled by anelectric filed applied to the liquid crystal (including a horizontalelectric field, a vertical electric field, and an oblique electricfield).

The following can be used for a liquid crystal element: a nematic liquidcrystal, a cholesteric liquid crystal, a smectic liquid crystal, adiscotic liquid crystal, a thermotropic liquid crystal, a lyotropicliquid Crystal, a low molecular liquid crystal, a polymer liquidcrystal, a ferroelectric liquid crystal, an anti-ferroelectric liquidcrystal, a main chain type liquid crystal, a side chain type polymerliquid crystal, a plasma addressed liquid crystal (PALC), abanana-shaped liquid crystal, a TN (Twisted Nematic) mode, an STN (SuperTwisted Nematic) mode, an IPS (In-Plane-Switching) mode, an FFS (FringeField Switching) mode, an MVA (Multi-domain Vertical Alignment) mode, aPVA (Patterned Vertical Alignment) mode, an ASV (Advanced Super View)mode, an ASM (Axially Symmetric aligned Microcell) mode, an OCB (OpticalCompensated Birefringence) mode, an ECB (Electrically ControlledBirefringence) mode, an FLC (Ferroelectric Liquid Crystal) mode, an AFLC(Anti-Ferroelectric Liquid Crystal) mode, a PDLC (Polymer DispersedLiquid Crystal) mode, and a guest-host mode.

Note that the invention is not limited thereto, and various kinds ofliquid crystal elements can be used.

Note that examples of electronic paper include a device displayed bymolecules which utilizes optical anisotropy, dye molecular orientationor the like; a device displayed by particles which utilizeselectrophoresis, particle movement, particle rotation, phase change, orthe like; a device displayed by moving one end of a film; a device usinglight emission or phase change of molecules; a device using opticalabsorption by molecules; and a device using self-light emission bycombining electrons and holes. For example, the following can be used aselectronic paper: a microcapsule type electrophoresis device, ahorizontal type electrophoresis device, a vertical type electrophoresisdevice, a device using a spherical twisting ball, a device using amagnetic twisting ball, a device using a column twisting ball, a deviceusing a charged toner, a quick-response liquid powder display, amagnetic electrophoresis type device, a magnetic heat-sensitive typedevice, an electrowetting type device, a light-scattering(transparent-opaque change) type device, a device using a cholestericliquid crystal and a photoconductive layer, a cholesteric liquid crystaldevice, a bistable nematic liquid crystal device, a ferroelectric liquidcrystal device, a liquid crystal dispersed type device with a dichroicdye, a device using a movable film, a device using coloring anddecoloring properties of a leuco dye, a photochromic device, anelectrochromic device, an electrodeposition device, a device usingflexible organic EL, and the like.

Note that the invention is not limited thereto, and various types ofelectronic paper can be used.

By using a microcapsule electrophoretic device, defects ofelectrophoresis, which are aggregation and precipitation of phoresisparticles, can be solved. Quick-response liquid powder has advantagessuch as high-speed response, high reflectivity, wide viewing angle, lowpower consumption, and memory properties.

A plasma display includes a substrate having a surface provided with anelectrode, and a substrate having a surface provided with an electrodeand a minute groove in which a phosphor layer is formed. In the plasmadisplay, the substrates are opposite to each other with a narrowinterval and a rare gas is sealed therein. Voltage is applied to theelectrodes to generate an ultraviolet ray so as to excite the phosphor;thus, display can be performed. The plasma display panel may be a DCtype PDP or an AC type PDP. As a driving method of the plasma displaypanel, ASW (Address While Sustain) driving, ADS (Address DisplaySeparated) driving in which a subframe is divided into a reset period,an address period, and a sustain period, CLEAR (High-Contrast, LowEnergy Address and Reduction of False Contour Sequence) driving, ALIS(Alternate Lighting of Surfaces) method, TERES (Technology of ReciprocalSustainer) driving, and the like can be used. Note that the invention isnot limited thereto, and various types of plasma displays can be used.

Note that electroluminescence, a cold cathode fluorescent lamp, a hotcathode fluorescent lamp, an LED, a laser light source, a mercury lamp,or the like can be used as a light source needed for a display device,such as a liquid crystal display device (a transmissive liquid crystaldisplay, a transflective liquid crystal display, a reflective liquidcrystal display, a direct-view liquid crystal display, and a projectiontype liquid crystal display), a display device using a grating lightvalve (GLV), and a display device using a digital micromirror device(DMD). Note that the invention is not limited thereto, and various lightsources can be used.

Note that as a transistor, various types of transistors can be employedwithout being limited to a certain type. For example, a thin filmtransistor (TFT) including a non-single crystalline semiconductor filmtypified by amorphous silicon, polycrystalline silicon, microcrystalline(also referred to as semi-amorphous) silicon, or the like can be used.The use of the TFT has various advantages. For example, since the TFTcan be formed at temperature lower than that of the case of using singlecrystalline silicon, reduction in manufacturing cost or increase in sizeof a manufacturing device can be realized. A transistor can be formedusing a large substrate with increase in size of the manufacturingdevice. Therefore, a large number of display devices can be formed atthe same time, and thus can be formed at low cost. Further, sincemanufacturing temperature is low, a substrate having low heat resistancecan be used. Accordingly, a transistor can be formed over alight-transmitting substrate; thus, transmission of light in a displayelement can be controlled by using the transistor formed over thelight-transmitting substrate. Alternatively, since the thickness of thetransistor is thin, part of a film forming the transistor can transmitlight; thus, an aperture ratio can be increased.

The use of a catalyst (e.g., nickel) when polycrystalline silicon isformed enables further improvement in crystallinity and formation of atransistor having excellent electrical characteristics. Thus, a gatedriver circuit (a scan line driver circuit), a source driver circuit (asignal line driver circuit), and a signal processing circuit (e.g., asignal generation circuit, a gamma correction circuit, a DA convertercircuit) can be formed over the same substrate.

The use of a catalyst (e.g., nickel) when microcrystalline silicon isformed enables further improvement in crystallinity and formation of atransistor having excellent electrical characteristics. At this time,the crystallinity can be improved by performing only heat treatmentwithout laser irradiation. Thus, a gate driver circuit (a scan linedriver circuit) and part of a source driver circuit (e.g., an analogswitch) can be formed over the same substrate. Further, when laserirradiation is not performed for crystallization, unevenness of siliconcrystallinity can be suppressed. Therefore, an image with high imagequality can be displayed.

Note that polycrystalline silicon and microcrystalline silicon can beformed without using a catalyst (e.g., nickel).

Note that the crystallinity of silicon is preferably improved topolycrystal or microcrystal in the whole panel, but not limited thereto.

The crystallinity of silicon may be improved only in part of the panel.The selective increase in crystallinity can be achieved by selectivelaser irradiation or the like.

For example, only a peripheral driver circuit region excluding pixelsmay be irradiated with laser light.

Alternatively, only a region of a gate driver circuit, a source drivercircuit, or the like may be irradiated with laser light.

Further alternatively, only part of a source driver circuit (e.g., ananalog switch) may be irradiated with laser light.

As a result, the crystallinity of silicon only in a region necessary forhigh-speed operation of a circuit can be improved.

A pixel region is not especially needed to operate at high speed. Thus,even if the crystallinity is not improved, the pixel circuit can operatewithout problems.

Thus, since a region crystallinity of which is improved is small,manufacturing steps can be shortened, throughput can be increased, andmanufacturing cost can be reduced.

Since the number of manufacturing devices needed is small, manufacturingcost can be reduced.

In addition, a transistor can be formed using a semiconductor substrate,an SOI substrate, or the like. Therefore, a small transistor with fewvariations in characteristics, sizes, shapes, or the like, with highcurrent supply capacity can be formed. By using such a transistor,reduction in power consumption or high integration of circuits can berealized.

A transistor including a compound semiconductor or an oxidesemiconductor such as ZnO, a-InGaZnO, SiGe, GaAs, IZO, ITO, or SnO, athin film transistor obtained by thinning such a compound semiconductoror a oxide semiconductor, or the like can be used. Therefore,manufacturing temperature can be lowered and for example, such atransistor can be formed at room temperature. Accordingly, thetransistor can be formed directly on a substrate having low heatresistance, such as a plastic substrate or a film substrate. Note thatsuch a compound semiconductor or an oxide semiconductor can be used fornot only a channel portion of the transistor but also otherapplications. For example, such a compound semiconductor or an oxidesemiconductor can be used as a resistor, a pixel electrode, or anelectrode having a light-transmitting property. Further, since such anelement can be formed at the same time as the transistor, cost can bereduced.

A transistor or the like formed by using an ink-jet method or a printingmethod can also be used. Accordingly, the transistor can be formed atroom temperature or at a low vacuum, or can be formed using a largesubstrate. Since the transistor can be formed without using a mask(reticle), layout of the transistor can be easily changed. Further,since it is not necessary to use a resist, material cost is reduced andthe number of steps can be reduced. Moreover, since a film is formedonly in a required portion, a material is not wasted and cost can bereduced compared with a manufacturing method in which etching isperformed after the film is formed over the entire surface.

A transistor or the like including an organic semiconductor or a carbonnanotube can also be used. Accordingly, a transistor can be formed usinga substrate which can be bent, and thus can resist a shock.

In addition, transistors with various structures can be used.

For example, a MOS transistor, a junction transistor, a bipolartransistor, or the like can be used as a transistor.

The use of a MOS transistor can reduce the size of a transistor.

Accordingly, a plurality of transistors can be mounted.

The use of a bipolar transistor can allow large current to flow; thus, acircuit can operate at high speed.

Further, a MOS transistor, a bipolar transistor, and the like may bemixed over one substrate.

Thus, low power consumption, reduction in size, and high-speed operationcan be achieved.

In addition, various other transistors can be used.

Note that transistor can be formed using various substrates. The type ofa substrate is not limited to a certain type. For example, a singlecrystalline substrate, an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, a stainless steel substrate, asubstrate including a stainless steel foil, or the like can be used as asubstrate. Alternatively, a skin (e.g., epidermis or corium) orhypodermal tissue of an animal such as a human may be used as thesubstrate. In addition, the transistor may be formed using onesubstrate, and then, the transistor may be transferred to anothersubstrate.

As a substrate to which the transistor is transferred, a singlecrystalline substrate, an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, a stainless steel substrate, asubstrate including a stainless steel foil, or the like can be used.

Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissueof an animal such as a human may be used as the substrate. In addition,a transistor may be formed using a substrate, and the substrate may bethinned by polishing. As a substrate to be polished, a singlecrystalline substrate, an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, a stainless steel substrate, asubstrate including a stainless steel foil, or the like can be used.

Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissueof an animal such as a human may be used as the substrate. By using sucha substrate, a transistor with excellent properties, a transistor withlow power consumption can be formed, a device with high durability canbe formed, high heat resistance can be provided, and reduction in weightor size can be realized.

Note that a structure of a transistor can employ various modes withoutbeing limited to a certain structure. For example, a multi-gatestructure having two or more gate electrodes may be employed. When themulti-gate structure is used, a structure where a plurality oftransistors are connected in series is provided since channel regionsare connected in series. The multi-gate structure realizes reduction inoff-current and improvement in reliability due to improvement inwithstand voltage of the transistor. Further, by using the multi-gatestructure, drain-source current does not change much even ifdrain-source voltage changes when the transistor operates in asaturation region; thus, the slope of voltage-current characteristicscan be flat. By utilizing the characteristics in which the slope of thevoltage-current characteristics is flat, an ideal current source circuitand an active load having an extremely high resistance value can berealized. Thus, a differential circuit or a current mirror circuithaving excellent properties can be realized. As another example, astructure where gate electrodes are formed above and below a channel maybe employed. By using the structure where gate electrodes are formedabove and below the channel, a channel region is enlarged, the amount ofcurrent can be increased since the number of channel regions isincreased, or a subthreshold swing can be reduced since a depletionlayer is easily formed. In the structure where the gate electrodes areformed above and below the channel, it seems that a plurality oftransistors are connected in parallel.

A structure where a gate electrode is formed above a channel region, astructure where a gate electrode is formed below a channel region, astaggered structure, an inversely staggered structure, a structure wherea channel region is divided into a plurality of regions, or a structurewhere channel regions are connected in parallel or in series can beemployed. In addition, a source electrode or a drain electrode mayoverlap with a channel region (or part thereof). By using the structurewhere the source electrode or the drain electrode may overlap with thechannel region (or part thereof), an unstable operation due toaccumulation of charge in part of the channel region can be prevented.Further, a structure where an LDD region is provided may be employed. Byproviding the LDD region, off-current can be reduced or the withstandvoltage of the transistor can be increased to improve reliability.Alternatively, drain-source current does not fluctuate much even ifdrain-source voltage fluctuates when the transistor operates in thesaturation region, so that characteristics where a slope ofvoltage-current characteristics is flat can be obtained.

Note that various types of transistors can be used for a transistor, andthe transistor can be formed using various types of substrates.Accordingly, all of circuits which are necessary to realize apredetermined function can be formed using the same substrate. Forexample, all of the circuits which are necessary to realize thepredetermined function can be formed using various substrates such as aglass substrate, a plastic substrate, a single crystalline substrate, oran SOI substrate. When all of the circuits which are necessary torealize the predetermined function are formed using the same substrate,cost can be reduced by reduction in the number of component parts orreliability can be improved by reduction in the number of connectionsbetween circuit components. Alternatively, part of the circuits whichare necessary to realize the predetermined function may be formed usingone substrate and another part of the circuits which are necessary torealize the predetermined function may be formed using anothersubstrate. That is, not all of the circuits which are necessary torealize the predetermined function are required to be formed using thesame substrate. For example, part of the circuits which are necessary torealize the predetermined function may be formed using transistors overa glass substrate and another part of the circuits which are necessaryto realize the predetermined function may be formed using a singlecrystalline substrate, so that an IC chip formed by a transistor on thesingle crystalline substrate can be connected to the glass substrate byCOG (Chip On Glass) and the IC chip may be provided over the glasssubstrate. Alternatively, the IC chip can be connected to the glasssubstrate by TAB (Tape Automated Bonding) or a printed wiring board.When part of the circuits are formed using the same substrate in thismanner, cost can be reduced by reduction in the number of componentparts or reliability can be improved by reduction in the number ofconnections between circuit components. Alternatively, since circuits ina portion with high driving voltage or a portion with high drivingfrequency consume large power, the circuits in such portions are formedusing a single crystalline substrate and using an IC chip formed by thecircuit instead of using the same substrate, for example; thus, increasein power consumption can be prevented.

Note that one pixel corresponds to one element brightness of which canbe controlled. For example, one pixel corresponds to one color elementand brightness is expressed with the one color element. Accordingly, inthe case of a color display device having color elements of R (Red), G(Green), and B (Blue), the smallest unit of an image is formed of threepixels of an R pixel, a G pixel, and a B pixel. Note that the colorelements are not limited to three colors, and color elements of morethan three colors may be used and/or a color other than RGB may be used.For example, RGBW can be employed by adding W (white). In addition, RGBadded with one or more colors of yellow, cyan, magenta emerald green,vermilion, and the like may be used. Further, a color similar to atleast one of R, G, and B may be added to RGB. For example, R, G, B1, andB2 may be used. Although both B1 and B2 are blue, they have slightlydifferent frequency. Similarly, R1, R2, G, and B may be used. By usingsuch color elements, display which is closer to the real object can beperformed and power consumption can be reduced. As another example, inthe case of controlling brightness of one color element by using aplurality of regions, one region may correspond to one pixel. Forexample, in the case of performing area ratio gray scale display or thecase of including a subpixel, a plurality of regions which controlbrightness are provided in each color element and gray scales areexpressed with all of the regions, and one region which controlsbrightness may correspond to one pixel. In that case, one color elementincludes a plurality of pixels. Alternatively, even when the pluralityof the regions which control brightness are provided in one colorelement, these regions may be collected and one color element may bereferred to as one pixel. In that case, one color element includes onepixel. Further, when brightness is controlled by a plurality of regionsin one color element, regions which contribute to display may havedifferent area dimensions depending on pixels. Alternatively, in theplurality of the regions which control brightness in one color element,signals supplied to the regions may be slightly varied to widen aviewing angle. That is, potentials of pixel electrodes included in theplurality of the regions in one color element may be different from eachother. Accordingly, voltages applied to liquid crystal molecules arevaried depending on the pixel electrodes. Therefore, the viewing anglecan be widened.

Note that when it is explicitly described as one pixel (for threecolors), it corresponds to the case where three pixels of R, G, and Bare considered as one pixel. When it is explicitly described as onepixel (for one color), it corresponds to the case where the plurality ofthe regions are provided in each color element and collectivelyconsidered as one pixel.

Note that pixels are provided (arranged) in matrix in some cases. Here,description that pixels are provided (arranged) in matrix includes thecase where the pixels are arranged in a straight line and the case wherethe pixels are arranged in a jagged line, in a longitudinal direction ora lateral direction. For example, in the case of performing full colordisplay with three color elements (e.g., RGB), the following cases areincluded therein: the case where the pixels are arranged in stripes, thecase where dots of the three color elements are arranged in a deltapattern, and the case where dots of the three color elements areprovided in Bayer arrangement. Note that the color elements are notlimited to three colors, and color elements of more than three colorsmay be employed, for example, RGBW (W corresponds to white), RGB addedwith one or more of yellow, cyan, magenta, and the like, or the like.Note that the size of display regions may be different betweenrespective dots of color elements. Thus, power consumption can bereduced or the life of a light-emitting element can be prolonged.

Note that an active matrix method in which an active element is includedin a pixel or a passive matrix method in which an active element is notincluded in a pixel can be used.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also various active elements(non-linear elements), for example, a MIM (Metal Insulator Metal), a TFD(Thin Film Diode), or the like can be used. Since such an element hasfew number of manufacturing steps, manufacturing cost can be reduced ora yield can be improved. Since the size of the element is small, anaperture ratio can be increased, and power consumption can be reducedand high luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is small, so that manufacturingcost can be reduced or the yield can be improved. Further, since anactive element (a non-linear element) is not used, the aperture ratiocan be increased, and power consumption can be reduced and highluminance can be achieved.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor includes a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor may change depending on astructure, operating conditions, and the like of the transistor, it isdifficult to define which is a source or a drain. Therefore, in thisdocument (the specification, the claim, the drawing, and the like), aregion functioning as a source and a drain is not called the source orthe drain in some cases. In such a case, one of the source and the drainmay be referred to as a first terminal and the other thereof may bereferred to as a second terminal, for example. Alternatively, one of thesource and the drain may be referred to as a first electrode and theother thereof may be referred to as a second electrode. Furtheralternatively, one of the source and the drain may be referred to as asource region and the other thereof may be referred to as a drainregion.

In addition, a transistor may be an element having at least threeterminals of a base, an emitter, and a collector. In this case also, oneof the emitter and the collector may be referred to as a first terminaland the other terminal may be referred to as a second terminal.

A gate corresponds to all or part of a gate electrode and a gate wiring(also referred to as a gate line, a gate signal line, a scan line, ascan signal line, or the like). A gate electrode corresponds to aconductive film which overlaps with a semiconductor forming a channelregion with a gate insulating film interposed therebetween. Note thatpart of the gate electrode overlaps with an LDD (Lightly Doped Drain)region or the source region (or the drain region) with the gateinsulating film interposed therebetween in some cases. A gate wiringcorresponds to a wiring for connecting a gate electrode of eachtransistor to each other, a wiring for connecting a gate electrodeincluded in each pixel to each other, or a wiring for connecting a gateelectrode to another wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) which functions as both a gate electrode and a gate wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a gate electrode or a gate wiring. That is, there is aregion where a gate electrode and a gate wiring cannot be clearlydistinguished from each other. For example, in the case where a channelregion overlaps with part of an extended gate wiring, the overlappedportion (region, conductive film, wiring, or the like) functions as botha gate wiring and a gate electrode. Accordingly, such a portion (aregion, a conductive film, a wiring, or the like) may be called either agate electrode or a gate wiring.

Note that a portion (a region, a conductive film, a wiring, or the like)which is formed of the same material as a gate electrode and forms thesame island as the gate electrode to be connected to the gate electrodemay also be called a gate electrode. Similarly, a portion (a region, aconductive film, a wiring, or the like) which is formed of the samematerial as a gate wiring and forms the same island as the gate wiringto be connected to the gate wiring may also be called a gate wiring. Ina strict sense, such a portion (a region, a conductive film, a wiring,or the like) does not overlap with a channel region or does not have afunction to connect the gate electrode to another gate electrode in somecases. However, there is a portion (a region, a conductive film, awiring, or the like) which is formed of the same material as a gateelectrode or a gate wiring and forms the same island as the gateelectrode or the gate wiring to be connected to the gate electrode orthe gate wiring in relation to a circuit structure and the like. Thus,such a portion (a region, a conductive film, a wiring, or the like) mayalso be called either a gate electrode or a gate wiring.

In a multi-gate transistor, for example, a gate electrode of onetransistor is often connected to a gate electrode of another transistorby using a conductive film which is formed of the same material as thegate electrode. Since such a portion (a region, a conductive film, awiring, or the like) is a portion (a region, a conductive film, awiring, or the like) for connecting the gate electrode and another gateelectrode, it may be called a gate wiring, and it may also be called agate electrode since a multi-gate transistor can be considered as onetransistor. That is, a portion (a region, a conductive film, a wiring,or the like) which is formed of the same material as a gate electrode ora gate wiring and forms the same island as the gate electrode or thegate wiring to be connected to the gate electrode or the gate wiring maybe called either a gate electrode or a gate wiring. In addition, part ofa conductive film which connects the gate electrode and the gate wiringand is formed of a material different from that of the gate electrodeand the gate wiring may also be called either a gate electrode or a gatewiring.

Note that a gate terminal corresponds to part of a portion (a region, aconductive film, a wiring, or the like) of a gate electrode or a portion(a region, a conductive film, a wiring, or the like) which iselectrically connected to the gate electrode.

When a gate electrode is called a gate wiring, a gate line, a gatesignal line, a scan line, a scan signal line, or the like, there is thecase where a gate of a transistor is not connected to a wiring. In thiscase, the gate wiring, the gate line, the gate signal line, the scanline, or the scan signal line corresponds to a wiring formed in the samelayer as the gate of the transistor, a wiring formed of the samematerial of the gate of the transistor, or a wiring formed at the sametime as the gate of the transistor in some cases. As examples, a wiringfor storage capacitance, a power supply line, a reference potentialsupply line, and the like can be given.

A source corresponds to all or part of a source region, a sourceelectrode, and a source wiring (also referred to as a source line, asource signal line, a data line, a data signal line, or the like). Asource region corresponds to a semiconductor region containing a largeamount of p-type impurities (e.g., boron or gallium) or n-typeimpurities (e.g., phosphorus or arsenic). Accordingly, a regioncontaining a small amount of p-type impurities or n-type impurities,namely, an LDD (Lightly Doped Drain) region is not included in thesource region. A source electrode is part of a conductive layer formedof a material different from that of a source region and electricallyconnected to the source region. However, there is the case where asource electrode and a source region are collectively called a sourceelectrode. A source wiring is a wiring for connecting a source electrodeof each transistor to each other, a wiring for connecting a sourceelectrode of each pixel to each other, or a wiring for connecting asource electrode to another wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) functioning as both a source electrode and a source wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring. That is, thereis a region where a source electrode and a source wiring cannot beclearly distinguished from each other. For example, in the case where asource region overlaps with part of an extended source wiring, theoverlapped portion (region, conductive film, wiring, or the like)functions as both a source wiring and a source electrode. Accordingly,such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring.

A portion (a region, a conductive film, a wiring, or the like) which isformed of the same material as a source electrode and forms the sameisland as the source electrode to be connected to the source electrode,or a portion (a region, a conductive film, a wiring, or the like) whichconnects a source electrode and another source electrode may also becalled a source electrode. Further, a portion which overlaps with asource region may be called a source electrode. Similarly, a regionwhich is formed of the same material as a source wiring and forms thesame island as the source wiring to be connected to the source wiringmay also be called a source wiring. In a strict sense, such a portion (aregion, a conductive film, a wiring, or the like) does not overlap witha channel region or does not have a function to connect the sourceelectrode to another source electrode in some cases. However, there is aportion (a region, a conductive film, a wiring, or the like) which isformed of the same material as a source electrode or a source wiring andforms the same island as the source electrode or the source wiring to beconnected to the source electrode or the source wiring in relation to acircuit structure and the like. Thus, such a portion (a region, aconductive film, a wiring, or the like) may also be called either asource electrode or a source wiring.

For example, part of a conductive film which connects a source electrodeand a source wiring and is formed of a material which is different fromthat of the source electrode or the source wiring may be called either asource electrode or a source wiring.

A source terminal corresponds to part of a source region, a sourceelectrode, or a portion (a region, a conductive film, a wiring, or thelike) which is electrically connected to the source electrode.

When a source electrode is called a source wiring, a source line, asource signal line, a data line, a data signal line, or the like, thereis the case in which a source (a drain) of a transistor is not connectedto a wiring. In this case, the source wiring, the source line, thesource signal line, the data line, or the data signal line correspondsto a wiring formed in the same layer as the source (drain) of thetransistor, a wiring formed of the same material of the source (drain)of the transistor, or a wiring formed at the same time as the source(drain) of the transistor in some cases. As examples, a wiring forstorage capacitance, a power supply line, a reference potential supplyline, and the like can be given.

Note that a drain is similar to the source.

Note that a semiconductor device corresponds to a device having acircuit including a semiconductor element (e.g., a transistor, a diode,or a thyristor). The semiconductor device may also refer to all deviceswhich can function by utilizing semiconductor characteristics.Alternatively, the semiconductor device refers to a device including asemiconductor material.

A display element corresponds to an optical modulation element, a liquidcrystal element, a light-emitting element, an EL element (an organic ELelement, an inorganic EL element, or an EL element including bothorganic and inorganic materials), an electron emitter, anelectrophoresis element, a discharging element, a light-reflectingelement, a light diffraction element, a digital micro device (DMD), orthe like. Note that the present invention is not limited thereto.

A display device corresponds to a device including a display element.The display device may include a plurality of pixels having a displayelement. The display device may include a peripheral driver circuit fordriving a plurality of pixels. The peripheral driver circuit for drivinga plurality of pixels may be formed over the same substrate as theplurality of pixels. In addition, the display device may also include aperipheral driver circuit provided over a substrate by wire bonding orbump bonding, namely, an IC chip connected by so-called chip on glass(COG), TAB, or the like. Further, the display device may also include aflexible printed circuit (FPC) to which an IC chip, a resistor, acapacitor, an inductor, a transistor, or the like is attached. Thedisplay device may also include a printed wiring board (PWB) which isconnected through a flexible printed circuit (FPC) and to which an ICchip, a resistor, a capacitor, an inductor, a transistor, or the like isattached. The display device may also include an optical sheet such as apolarizing plate or a retardation plate. The display device may alsoinclude a lighting device, a housing, an audio input and output device,a light sensor, or the like. Here, a lighting device such as a backlightunit may include a light guide plate, a prism sheet, a diffusion sheet,a reflective sheet, a light source (e.g., an LED or a cold cathodefluorescent lamp), a cooling device (e.g., a water cooling device or anair cooling device), or the like.

A lighting device corresponds to a device including a backlight unit, alight guide plate, a prism sheet, a diffusion sheet, a reflective sheet,a light source (e.g., an LED, a cold cathode fluorescent lamp, or a hotcathode fluorescent lamp), a cooling device, or the like.

A light-emitting device corresponds to a device including alight-emitting element or the like. A light-emitting device including alight-emitting element as a display element is a specific example of adisplay device.

A reflective device corresponds to a device including a light-reflectingelement, a light diffraction element, a light reflecting electrode, orthe like.

A liquid crystal display device corresponds to a display deviceincluding a liquid crystal element. Liquid crystal display devicesinclude a direct-view liquid crystal display, a projection liquidcrystal display, a transmissive liquid crystal display, a reflectiveliquid crystal display, a transflective liquid crystal display, and thelike.

A driving device corresponds to a device including a semiconductorelement, an electric circuit, or an electronic circuit. Examples of thedriving device include a transistor (also referred to as a selectiontransistor, a switching transistor, or the like) which controls input ofa signal from a source signal line to a pixel, a transistor whichsupplies voltage or current to a pixel electrode, a transistor whichsupplies voltage or current to a light-emitting element, and the like.Moreover, examples of the driving device include a circuit (alsoreferred to as a gate driver, a gate line driver circuit, or the like)which supplies a signal to a gate signal line, a circuit (also referredto as a source driver, a source line driver circuit, or the like) whichsupplies a signal to a source signal line, and the like.

A display device, a semiconductor device, a lighting device, a coolingdevice, a light-emitting device, a reflective device, a driving device,and the like overlap with each other in some cases. For example, adisplay device includes a semiconductor device and a light-emittingdevice in some cases. Further, a semiconductor device includes a displaydevice and a driving device in some cases.

When it is explicitly described that B is formed on A or that B isformed over A, it does not necessarily mean that B is formed in directcontact with A. The description includes the case where A and B are notin direct contact with each other, that is, the case where anotherobject is interposed between A and B. Here, each of A and B correspondsto an object (e.g., a device, an element, a circuit, a wiring, anelectrode, a terminal, a conductive film, or a layer).

For example, when it is explicitly described that a layer B is formed on(or over) a layer A, it includes both the case where the layer B isformed in direct contact with the layer A, and the case where anotherlayer (e.g., a layer C or a layer D) is formed in direct contact withthe layer A and the layer B is formed in direct contact with the layer Cor D. Note that another layer (e.g., a layer C or a layer D) may be asingle layer or a plurality of layers.

Similarly, when it is explicitly described that B is formed above A, itdoes not necessarily mean that B is in direct contact with A, andanother object may be interposed between A and B. For example, when itis explicitly described that a layer B is formed above a layer A, itincludes both the case where the layer B is formed in direct contactwith the layer A, and the case where another layer (e.g., a layer C or alayer D) is formed in direct contact with the layer A and the layer B isformed in direct contact with the layer C or D. Note that another layer(e.g., a layer C or a layer D) may be a single layer or a plurality oflayers.

When it is explicitly described that B is formed in direct contact withA, it does not include the case where another object is interposedbetween A and B and includes the case where B is formed in directcontact with A.

Note that the same can be said when it is explicitly described that B isformed below or under A.

Note that explicit singular forms are preferably singular forms.However, without being limited thereto, such singular forms can includeplural forms. Similarly, explicit plural forms are preferably pluralforms. However, without being limited thereto, such plural forms caninclude singular forms.

The structures applicable to a liquid crystal display device and asemiconductor device of the invention are described above. Thestructures described above may be applied to a liquid crystal displaydevice and a semiconductor device of the invention when needed.

According to the invention, a liquid crystal display device with a wideviewing angle and lower manufacturing cost than a conventional liquidcrystal display device can be provided.

Since a conductive film is formed over an entire surface of a substratein the invention, an impurity from the substrate can be prevented frombeing mixed into an active layer. Thus, a liquid crystal display devicewith high reliability and a semiconductor device including the liquidcrystal display device can be obtained.

In the invention, when a semiconductor device including a top-gate thinfilm transistor is formed, a potential of a back gate is stabilized;thus, a liquid crystal display device with high reliability and asemiconductor device including the liquid crystal display device can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a structure example of a pixelportion using a top-gate thin film transistor.

FIG. 2 is a cross-sectional view showing a structure example of a pixelportion using a bottom-gate thin film transistor.

FIG. 3 is a cross-sectional view showing a structure example of a pixelportion using a top-gate thin film transistor.

FIG. 4 is a top plan view of the pixel portion shown in FIGS. 1 and 3.

FIG. 5 is a cross-sectional view of a liquid crystal display device ofthe invention.

FIG. 6 is a cross-sectional view of a liquid crystal display device ofthe invention.

FIG. 7 is a top plan view of a liquid crystal display device of theinvention.

FIGS. 8A to 8D are top plan views of a liquid crystal display device ofthe invention.

FIGS. 9A to 9D are top plan views of a liquid crystal display device ofthe invention.

FIG. 10 is a cross-sectional view of a liquid crystal display device ofthe invention.

FIG. 11 is a top plan view of a liquid crystal display device of theinvention.

FIG. 12 is a cross-sectional view of a liquid crystal display device ofthe invention.

FIG. 13 is a top plan view of a liquid crystal display device of theinvention.

FIG. 14A is a top plan view of a liquid crystal display device of theinvention, and FIG. 14B is a cross-sectional view thereof.

FIGS. 15A to 15D are cross-sectional views showing manufacturing stepsof a liquid crystal display device of the invention.

FIGS. 16A to 16C are cross-sectional views showing manufacturing stepsof a liquid crystal display device of the invention.

FIGS. 17A to 17C are cross-sectional views showing manufacturing stepsof a liquid crystal display device of the invention.

FIG. 18 is a cross-sectional view showing a manufacturing step of aliquid crystal display device of the invention.

FIGS. 19A and 19B are circuit diagrams of a liquid crystal displaydevice of the invention.

FIGS. 20A and 20B are circuit diagrams of a liquid crystal displaydevice of the invention.

FIGS. 21A to 21H show examples of electronic devices each formed byusing a liquid crystal display device of the invention.

FIG. 22 is a cross-sectional view of a liquid crystal display device ofthe invention.

FIGS. 23A and 23B are top plan views of a liquid crystal display deviceof the invention.

FIG. 24 is a cross-sectional view of a liquid crystal display device ofthe invention.

FIG. 25 is a cross-sectional view of a liquid crystal display device ofthe invention.

FIG. 26 is a cross-sectional view of a liquid crystal display device ofthe invention.

FIG. 27 is a top plan view of a liquid crystal display device of theinvention.

FIG. 28 shows a liquid crystal display device of the invention.

FIGS. 29A to 29G are cross-sectional views showing transistors accordingto the invention.

FIG. 30 is a cross-sectional view showing a transistor according to theinvention.

FIG. 31 is a cross-sectional view showing a transistor according to theinvention.

FIG. 32 is a cross-sectional view showing a transistor according to theinvention.

FIG. 33 is a cross-sectional view showing a transistor according to theinvention.

FIGS. 34A to 34C show structures of a display device according to theinvention.

FIGS. 35A and 35B show structures of a display device according to theinvention.

FIG. 36 shows a structure of a display device according to theinvention.

FIG. 37 shows one driving method of a display device according to theinvention.

FIG. 38 shows one driving method of a display device according to theinvention.

FIGS. 39A and 39B each show one driving method of a display deviceaccording to the invention.

FIG. 40 is a cross-sectional view of a liquid crystal display deviceaccording to the invention.

FIGS. 41A to 41D are cross-sectional views of a liquid crystal displaydevice according to the invention.

FIG. 42 is a cross-sectional view of a liquid crystal display deviceaccording to the invention.

FIGS. 43A to 43C each show a structure of a liquid crystal displaydevice according to the invention.

FIG. 44 is a cross-sectional view of a liquid crystal display deviceaccording to the invention.

FIGS. 45A and 45B are circuit diagrams of a pixel according to theinvention.

FIG. 46 is a circuit diagram of a pixel according to the invention.

FIG. 47 is a circuit diagram of a pixel according to the invention.

FIGS. 48A to 48E each show one driving method of a liquid crystaldisplay device according to the invention.

FIGS. 49A and 49B each show one driving method of a liquid crystaldisplay device according to the invention.

FIGS. 50A to 50C each show one driving method of a liquid crystaldisplay device according to the invention.

FIGS. 51A to 51C each show one driving method of a liquid crystaldisplay device according to the invention.

FIGS. 52A to 52C each show one driving method of a liquid crystaldisplay device according to the invention.

FIGS. 53A and 53B show a structure of a display device according to theinvention.

FIG. 54 shows a structure of a display device according to theinvention.

FIG. 55 shows a structure of a display device according to theinvention.

FIG. 56 shows a structure of a display device according to theinvention.

FIGS. 57A to 57C each show a structure of a display device according tothe invention.

FIG. 58 shows an electronic device according to the invention.

FIG. 59 shows an electronic device according to the invention.

FIGS. 60A and 60B each show an electronic device according to theinvention.

FIG. 61 shows an electronic device according to the invention.

FIGS. 62A to 62C each show an electronic device according to theinvention.

FIG. 63 shows an electronic device according to the invention.

FIG. 64 shows an electronic device according to the invention.

FIG. 65 shows an electronic device according to the invention.

FIG. 66 shows an electronic device according to the invention.

FIGS. 67A and 67B show an electronic device according to the invention.

FIGS. 68A and 68B show an electronic device according to the invention.

FIGS. 69A to 69C each show an electronic device according to theinvention.

FIGS. 70A and 70B each show an electronic device according to theinvention.

FIG. 71 shows an electronic device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes of the present invention will be describedwith reference to drawings. However, the present invention can beimplemented in various modes, and it is easily understood by thoseskilled in the art that modes and details can be variously changedwithout departing from the scope and the spirit of the presentinvention. Therefore, the present invention is not construed as beinglimited to description of the embodiment modes. Note that in thedrawings shown below, the same portions or portions having similarfunctions are denoted by the same reference numerals, and repeateddescription is omitted.

Embodiment Mode 1

This embodiment mode is described with reference to FIGS. 1 and 3 to 5.

FIG. 1 shows an example where a top-gate thin film transistor (TFT) isused as a switching element in a pixel portion. A conductive film 115 tobe a first electrode in FFS (Fringe-Field Switching) drive is formedover an entire surface of one side of a substrate 101.

A light-transmitting conductive film is used as the conductive film 115.As such a light-transmitting conductive film, an indium tin oxide (ITO)film, an indium zinc oxide (IZO) film, an indium tin oxide containingsilicon (also referred to as ITSO) film, a zinc oxide (ZnO) film, acadmium tin oxide (CTO) film, a tin oxide (SnO) film, or the like may beused.

A base film 102 is formed over the conductive film 115, and a thin filmtransistor (TFT) 121 is formed over the base film 102. The TFT 121includes a region 131 a which is one of a source region and a drainregion, a region 131 b which is the other of the source region and thedrain region, an active layer 103 including a channel formation region132, a gate insulating film 104, and a gate electrode 105. Note thatalthough the gate insulating film 104 in FIG. 1 is formed only above thechannel formation region 132, it may be formed over portions other thanabove the channel formation region 132.

An interlayer insulating film 106 is formed over the TFT 121 and thebase film 102. An electrode 107 (a source wiring) which is electricallyconnected to one of the source region and the drain region through acontact hole in the interlayer insulating film 106, and an electrode 108which is electrically connected to the other of the source region andthe drain region through a contact hole in the interlayer insulatingfilm 106 are formed over the interlayer insulating film 106.

An interlayer insulating film 111 is formed over the interlayerinsulating film 106, the electrodes 107 and 108, and an electrode 109.Further, pixel electrodes 113 and 114 a to 114 c which are electricallyconnected to the electrode 108 through a contact hole formed in theinterlayer insulating film 111 are formed over the interlayer insulatingfilm 111. Note that the pixel electrode 113 may be electricallyconnected to the electrode 107, not to the electrode 108. In addition,only one of the interlayer insulating films 106 and 111 may be formed.

As shown in FIG. 1, an electric field 125 is generated between the pixelelectrodes 113 and 114 (114 a to 114 c) and the conductive film 115. Asdescribed below, liquid crystal molecules are driven by the electricfield 125.

Further, as shown in FIG. 3, the conductive film 115 is electricallyconnected to the connection electrode 109 through a contact hole in theinterlayer insulating film 106 and the base film 102, and the connectionelectrode 109 is electrically connected to a wiring 119. Note that thewiring 119 is formed of the same material and in the same step as thegate electrode 105. The connection electrode 109 is formed of the samematerial and in the same step as the electrodes 107 and 108. Thus, theycan be formed without adding a manufacturing step, so that the number ofphotomasks can be reduced. Note that the same portions in FIGS. 1 and 3are denoted by the same reference numerals.

Note that the wiring 119 may be arranged in parallel to the gateelectrode 105. When the wiring 119 is arranged in parallel to the gateelectrode 105, decrease in aperture ratio is reduced.

When the wiring 119 is connected to the conductive film 115 for eachpixel, resistance of the conductive film 115 can be reduced. Further,waveform distortion can be reduced.

In addition, the connection electrode 109 may be extended across thepixels, without being connected to the wiring 119. In this case, theconnection electrode 109 is preferably arranged in parallel to thesource wiring 107.

FIG. 4 is a top plan view of FIGS. 1 and 3. FIG. 3 is a cross-sectionalview along A-A′ and B-B′ of FIG. 4. FIG. 1 is a cross-sectional viewalong A-A′ of FIG. 4. The pixel electrodes 113 and 114 a to 114 c, andthe like are provided with grooves (also referred to as openings, slits,apertures, gaps, or spaces) 117.

As shown in FIG. 4, a plurality of source wirings 107 are provided inparallel to each other (extended in a vertical direction in the drawing)and apart from each other. A plurality of gate wirings 105 are extendedin a direction generally perpendicular to the source wirings 107 (ahorizontal direction in the drawing) and provided apart from each other.The wiring 119 is adjacent to the plurality of gate wirings 105 andextended in a direction parallel to the gate wirings 105, that is, adirection perpendicular to the source wirings 107 (the horizontaldirection in the drawing). By such arrangement, an aperture ratio can beincreased. A space with a generally rectangular shape, which issurrounded by the source wirings 107, the wiring 119, and the gatewiring 105, is provided with the pixel electrode 113 of a liquid crystaldisplay device. The thin film transistor 121 for driving the pixelelectrode 113 is provided at an upper left corner of the drawing. Theplurality of pixel electrodes and thin film transistors are arranged inmatrix.

Note that in this embodiment mode, the wiring 119 and the conductivefilm 115 are connected to each pixel through contact holes; however, theinvention is not limited thereto.

Note that the wiring 119 is provided in FIG. 4; FIG. 27 shows an examplewhere the gate wiring 105 is used instead of the wiring 119. Across-sectional view of FIG. 27 is the same as FIG. 3, except that thewiring 119 is the same wiring as the gate wiring 105.

Note that each of the gate wiring 105, the wiring 119, and the sourcewiring 107 is preferably formed of one or more elements selected fromaluminum (Al), tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten(W), neodymium (Nd), chromium (Cr), nickel (Ni), platinum (Pt), gold(Au), silver (Ag), copper (Cu), magnesium (Mg), scandium (Sc), cobalt(Co), zinc (Zn), niobium (Nb), silicon (Si), phosphorus (P), boron (B),arsenic (As), gallium (Ga), indium (In), tin (Sn), and oxygen (O); or acompound or an alloy material including one or more of theaforementioned elements (e.g., indium tin oxide (ITO), indium zinc oxide(IZO), indium tin oxide containing silicon (ITSO), zinc oxide (ZnO), tinoxide (SnO), cadmium tin oxide (CTO), aluminum neodymium (Al—Nd),magnesium silver (Mg—Ag), or molybdenum-niobium (Mo—Nb)); a substance inwhich these compounds are combined; or the like. Alternatively, each ofthe gate wiring 105, the wiring 119, and the source wiring 107 ispreferably formed to contain a substance including a compound (silicide)of silicon and one or more of the aforementioned elements (e.g.,aluminum silicon, molybdenum silicon, or nickel silicide); or a compoundof nitrogen and one or more of the aforementioned elements (e.g.,titanium nitride, tantalum nitride, or molybdenum nitride).

Note that silicon (Si) may contain an n-type impurity (such asphosphorus) or a p-type impurity (such as boron). When silicon containsthe impurity, the conductivity is increased, and a function similar to ageneral conductor can be realized. Thus, such silicon can be utilizedeasily as a wiring, an electrode, or the like.

In addition, silicon with various levels of crystallinity, such assingle crystalline silicon, polycrystalline silicon, or microcrystallinesilicon can be used. Alternatively, silicon having no crystallinity,such as amorphous silicon can be used. By using single crystallinesilicon or polycrystalline silicon, resistance of a wiring, anelectrode, a conductive layer, a conductive film, a terminal, or thelike can be reduced. By using amorphous silicon or microcrystallinesilicon, a wiring or the like can be formed by a simple process.

Aluminum and silver have high conductivity, and thus can reduce a signaldelay. Further, since aluminum and silver can be easily etched, they canbe minutely processed.

Copper has high conductivity, and thus can reduce a signal delay. Whencopper is used, a stacked-layer structure is preferably employed toimprove adhesion.

Molybdenum and titanium are preferable since even if molybdenum ortitanium is in contact with an oxide semiconductor (e.g., ITO or IZO) orsilicon, molybdenum or titanium does not cause defects. Further,molybdenum and titanium are preferable since they are easily etched andhas high heat resistance.

Tungsten is preferable since it has an advantage such as high heatresistance.

Neodymium is also preferable since it has an advantage such as high heatresistance. In particular, an alloy of neodymium and aluminum ispreferable since heat resistance is increased and aluminum hardly causeshillocks.

Silicon is preferable since it can be formed at the same time as asemiconductor layer included in a transistor and has high heatresistance.

Since ITO, IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO),and cadmium tin oxide (CTO) have light-transmitting properties, they canbe used as a portion which transmits light. For example, they can beused for a pixel electrode or a common electrode.

IZO is preferable since it is easily etched and processed. In etchingIZO, a residue is hardly left. Thus, when IZO is used for a pixelelectrode, defects (such as short circuit or orientation disorder) of aliquid crystal element or a light-emitting element can be reduced.

A wiring, an electrode, a conductive layer, a conductive film, aterminal, or the like may have a single-layer structure or a multi-layerstructure. By employing a single-layer structure, each manufacturingprocess of a wiring, an electrode, a conductive layer, a conductivefilm, a terminal, or the like can be simplified, the number of days fora process can be reduced, and cost can be reduced. Alternatively, byemploying a multi-layer structure, a wiring, an electrode, and the likewith high quality can be formed while an advantage of each material isutilized and a disadvantage thereof is reduced. For example, when alow-resistant material (e.g., aluminum) is included in a multi-layerstructure, reduction in resistance of a wiring can be realized. Further,when a stacked-layer structure where a low heat-resistant material isinterposed between high heat-resistant materials is employed, heatresistance of a wiring, an electrode, and the like can be increased,utilizing advantages of the low heat-resistance material. For example,it is preferable to employ a stacked-layer structure where a layercontaining aluminum is interposed between layers containing molybdenum,titanium, neodymium, or the like.

When wirings, electrodes, or the like are in direct contact with eachother, they adversely affect each other in some cases. For example, onewiring or one electrode is mixed into a material of another wiring oranother electrode and changes its properties, and thus, an intendedfunction cannot be obtained in some cases. As another example, when ahigh-resistant portion is formed, a problem may occur so that it cannotbe normally formed. In such cases, a reactive material is preferablyinterposed by or covered with a non-reactive material in a stacked-layerstructure. For example, when ITO and aluminum are connected, titanium,molybdenum, or an alloy of neodymium is preferably interposed betweenITO and aluminum. As another example, when silicon and aluminum areconnected, titanium, molybdenum, or an alloy of neodymium is preferablyinterposed between silicon and aluminum.

Note that the term “wiring” indicates a portion including a conductor. Awiring may be provided linearly or may be short without being extendedlinearly. Therefore, an electrode is included in a wiring.

Note that the gate wiring 105 is preferably formed of a material withhigher heat resistance than that of the source wiring 107. This isbecause the gate wiring 105 is more likely to be exposed to a hightemperature in manufacturing steps than the source wiring 107.

In addition, the source wiring 107 is preferably formed of a materialwith lower resistance than that of the gate wiring 105. This is becauseonly signals with two values, which is an H-level signal and an L-levelsignal, are supplied to the gate wiring 105, whereas an analog signalwhich contributes to display is supplied to the source wiring 107.Accordingly, a material with low resistance is preferably used for thesource wiring 107 in order that an accurate signal is supplied to thesource wiring 107.

Note that although the wiring 119 does not have to be provided, apotential of a common electrode in each pixel can be stabilized when thewiring 119 is provided. Note also that the wiring 119 is provided inparallel to the gate wiring in FIG. 4; however, the invention is notlimited thereto. The wiring 119 may be provided in parallel to thesource wiring 107. At this time, the wiring 119 is preferably formed ofthe same material as the source wiring 107.

Since an aperture ratio can be increased and layout can be efficientlyperformed, the wiring is preferably parallel to the gate wiring.

The substrate 101 is a glass substrate, a quartz substrate, a substrateformed of an insulator such as alumina, a plastic substrate with heatresistance high enough to withstand a processing temperature ofsubsequent steps, a single crystalline substrate (e.g., a singlecrystalline silicon substrate), an SOI substrate, or a metal plate.Alternatively, the substrate 101 may be formed of polycrystallinesilicon.

When a display device operates as a transmissive display device, thesubstrate 101 is preferably has a light-transmitting property.

The conductive film 115 is formed of a conductive film with alight-transmitting property (e.g., an indium tin oxide (ITO) film, anindium zinc oxide (IZO) film, a zinc oxide (ZnO) film, a tin oxide (SnO)film, or a polycrystalline silicon film or an amorphous silicon filminto which an impurity is introduced).

An insulating film is formed as the base film 102 over the conductivefilm 115. The insulating film 102 is for preventing impurities from thesubstrate 101 from being diffused, and functions as the base film. Theinsulating film 102 is formed of an insulating material containingoxygen or nitrogen, such as silicon oxide (SiOx), silicon nitride(SiNx), silicon oxide containing nitrogen (SiOxNy: x>y), or siliconnitride containing oxygen (SiNxOy: x>y). Alternatively, the insulatingfilm 102 may be a stacked-layer film in which a plurality of these filmsare stacked. Note that an insulating film having the same function asthe insulating film 102 may be provided between the substrate 101 andthe conductive film 115.

For example, a stacked-layer film of a silicon nitride film and asilicon oxide film; or a single-layer film of a silicon oxide film maybe used as the base film 102. It is useful to use a silicon oxide filmwhich is thicker than the gate insulating film 104 as the base film 102because capacitive coupling with the gate wiring 105 can be reduced.Accordingly, the base film 102 is preferably thicker than, morepreferably three times thicker than, the gate insulating film 104.

The semiconductor film 103 is formed over the insulating film 102. Theregion 131 a to be one of the source region and the drain region, andthe region 131 b to be the other of the source region and the drainregion are formed in the semiconductor film 103. The regions 131 a and131 b are n-type impurity regions, for example, but may be p-typeimpurity regions. As an impurity imparting n-type conductivity,phosphorus (P) or arsenic (As) is used, for example. As an impurityimparting p-type conductivity, boron (B) or gallium (Ga) is used, forexample. The channel formation region 132 is formed between the regions131 a and 131 b.

Further, low concentration impurity regions may be formed between theregion 131 a and the channel formation region 132, and the region 131 band the channel formation region 132.

As shown in FIG. 4, the conductive film 115 is formed over almost theentire surface of the pixel. Each rectangular region surrounded by thesource wirings 107, the wiring 119, and the gate wiring 105, is providedwith the thin film transistor 121. That is, the gate wiring 105 isformed as a first wiring, the source wiring 107 is formed as a secondwiring, and the wiring 119 is formed as a third wiring. By provision ofthe thin film transistor 121, a region which is effective for display inthe pixel can be formed more efficiently. In other words, an apertureratio can be increased. Note that the semiconductor film 103 is apolycrystalline silicon film, for example, but may be anothersemiconductor film (e.g., an amorphous silicon film, a singlecrystalline silicon film, an organic semiconductor film, or a carbonnanotube) or a microcrystalline silicon film (also referred to as asemi-amorphous silicon film).

Here, a semi-amorphous semiconductor film typified by a semi-amorphoussilicon film includes a semiconductor having an intermediate structurebetween an amorphous semiconductor film and a semiconductor film havinga crystalline structure (including single crystal and polycrystalline).A semi-amorphous semiconductor film is a semiconductor film having athird state which is stable in free energy, is a crystalline substancewith a short-range order and lattice distortion, and can be dispersed ina non-single crystalline semiconductor film with a grain size of 0.5 to20 nm. Raman spectrum of a semi-amorphous semiconductor film shifts to awave number side lower than 520 cm⁻¹, and the diffraction peaks of (111)and (220) which are thought to be derived from a silicon crystallinelattice are observed by X-ray diffraction. Further, a semi-amorphoussemiconductor film contains hydrogen or halogen of at least 1 atomic %or more to terminate dangling bonds. In this specification, such asemiconductor film is referred to as a semi-amorphous semiconductor(SAS) film for convenience. When a rare gas element such as helium,argon, krypton, or neon is contained to further increase the latticedistortion, stability can be enhanced, and a favorable semi-amorphoussemiconductor film can be obtained.

In addition, a SAS film can be obtained by glow discharge decompositionof a gas containing silicon. As a typical gas containing silicon, SiH₄,or Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like can be used.Further, when the gas containing silicon is diluted with hydrogen orwith a gas in which one or more of rare gas elements of helium, argon,krypton, and neon are added to hydrogen, the SAS film can be easilyformed. The gas containing silicon is preferably diluted at a dilutionratio in the range of 2 to 1000 times. In addition, a carbide gas suchas CH₄ or C₂H₆; a germanium gas such as GeH₄ or GeF₄; F₂; or the likemay be mixed into the gas containing silicon to adjust the energybandwidth to be from 1.5 to 2.4 eV or 0.9 to 1.1 eV.

A semiconductor layer may be provided below the gate wiring 105. Thus,capacitive coupling between the conductive film 115 and the gate wiring105 can be reduced. Accordingly, the gate wiring 105 can be rapidlycharged and discharged, and waveform distortion can be suppressed.

The gate insulating film 104 of the thin film transistor 121 is formedover the semiconductor film 103.

Note that the gate insulating film 104 is provided only near the channelregion and not provided in other portions in some cases. Further,thickness or a stacked-layer structure of the gate insulating film 104may differ depending on a position. For example, the gate insulatingfilm 104 may be thicker or include more layers only near the channel andmay be thinner or include fewer layers in other portions. This makes iteasy to control the addition of an impurity to the source region and thedrain region. Further, when the thickness or the number of layers of thegate insulating film 104 near the channel differs, the amount ofimpurity added to the semiconductor film are different depending on aposition, so that an LDD region can be formed. When the LDD region isformed, leak current and generation of hot carriers can be suppressed,and reliability can be improved.

The gate insulating film 104 is not necessarily formed in a region inwhich the pixel electrode 113 is formed. In this case, a distancebetween the pixel electrode 113 and the conductive film 115 can bereduced, so that an electric field can be easily controlled.

The gate insulating film 104 is formed of, for example, an insulatingmaterial containing oxygen or nitrogen, such as silicon oxide (SiOx),silicon nitride (SiNx), silicon oxide containing nitrogen (SiOxNy: x>y),or silicon nitride containing oxygen (SiNxOy: x>y). Alternatively, thegate insulating film 104 may be a stacked-layer film in which aplurality of these films are stacked. The gate electrode 105 providedabove the semiconductor film 103 is formed over the gate insulating film104.

As shown in FIGS. 4 and 3, the gate electrode (the gate wiring) 105 isin the same wiring layer as the wiring 119.

The first interlayer insulating film 106 is formed over the gateinsulating film 104 and the gate electrode 105. An inorganic material oran organic material can be used for the first interlayer insulating film106. As the organic material, polyimide, acrylic, polyamide, polyimideamide, resist, siloxane, polysilazane, or the like can be used. As theinorganic material, an insulating material containing oxygen ornitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx), siliconoxide containing nitrogen (SiOxNy: x>y), or silicon nitride containingoxygen (SiNxOy: x>y); or a stacked-layer film in which a plurality ofthese films are stacked can be used. Alternatively, a stacked-layer filmin which the organic material and the inorganic material are combinedmay be used.

A contact hole located over the region 131 a, a contact hole locatedover the region 131 b, and a contact hole located over the wiring 119are formed in the insulating film 102, the gate insulating film 104, andthe first interlayer insulating film 106. The source wiring 107, theelectrode 108, and the connection electrode 109 are formed over thefirst interlayer insulating film 106.

Note that when an inorganic material is used for the insulating film,intrusion of moisture and an impurity can be prevented. In particular, alayer containing nitrogen can efficiently block moisture and animpurity.

Note that when an organic material is used for the insulating film, asurface thereof can be planarized. Accordingly, the insulating film canhave a good effect on a layer provided thereover. For example, the layerformed over the organic material can also be planarized, so thatdisturbance of orientation of liquid crystals can be prevented.

The source wiring 107 is located above the region 131 a, and iselectrically connected to the region 131 a through the contact hole.Therefore, the electrode 108 is electrically connected to the region 131b through the contact hole.

Note that the pixel electrode 113 and the impurity region 131 b may bedirectly connected to each other without a conductive film forconnection interposed therebetween. In this case, a contact hole forconnecting the pixel electrode 113 and the impurity region 131 b isneeded to be deep; however, a conductive film for connection is notneeded, and a region for the conductive film for connection can be usedas an opening region for displaying an image. Thus, an aperture ratiocan be increased, and reduction in power consumption can be realized.

The connection electrode 109 is located above the wiring 119, and iselectrically connected to the wiring 119 and the conductive film 115.The conductive film 115 is electrically connected to the wiring 119through the connection electrode 109 in this manner. Note that aplurality of connection electrodes 109 may be provided. Thus, apotential of the conductive film 115 is stabilized. Further, the numberof times for forming a contact hole can be reduced by connecting theconductive film 115 and the wiring 119 through the connection electrode109, so that a process can be simplified.

Note that the connection electrode 109 is formed of the same material asthe source wiring 107 at the same time; however, the invention is notlimited thereto. The connection electrode 109 may be formed of the samematerial as the pixel electrode 113 at the same time.

The second interlayer insulating film 111 is formed over the sourcewiring 107, the electrode 108, the connection electrode 109, and thefirst interlayer insulating film 106. Note that a structure may beemployed in which the second interlayer insulating film 111 is notformed (see FIG. 28). An inorganic material or an organic material canbe used for the second interlayer insulating film 111. As the organicmaterial, polyimide, acrylic, polyamide, polyimide amide, resist,siloxane, polysilazane, or the like can be used. As the inorganicmaterial, an insulating material containing oxygen or nitrogen, such assilicon oxide (SiOx), silicon nitride (SiNx), silicon oxide containingnitrogen (SiOxNy: x>y), or silicon nitride containing oxygen (SiNxOy:x>y); or a stacked-layer film in which a plurality of these films arestacked can be used. Alternatively, a stacked-layer film in which theorganic material and the inorganic material are combined may be used.

FIG. 28 is a cross-sectional view in the case where the secondinterlayer insulating film 111 is not formed. In FIG. 28, the portionssame as those in FIG. 3 are denoted by the same reference numerals.Since the electrode 108 is not formed, the pixel electrode 113 isdirectly connected to the island-shaped semiconductor film 103. Thesource wiring 107, the pixel electrodes 113 and 114, and the connectionelectrode 109 are formed of the same material in the same step. In astructure shown in FIG. 28, a distance between the pixel electrode 113and the conductive film 115 can be reduced, and an electric field can beeasily controlled.

The pixel electrodes 113, 114 a, 114 b, and 114 c, and the like whichare second electrodes in FFS drive are formed over the second interlayerinsulating film 111. Note that the pixel electrode 113 and the pixelelectrodes 114 (114 a, 114 b, 114 c, and the like) are separated inFIGS. 1 and 3, which are cross-sectional views, for convenience; as isapparent from FIG. 4, which is a top plan view, the pixel electrodes areformed in such a manner that grooves (also referred to as openings,slits, apertures, gaps, or spaces) 117 (117 a, 117 b, 117 c, and thelike) are formed in a conductive film formed of the same material in thesame step. Accordingly, in the following description, the pixelelectrodes 113 and 114 (114 a, 114 b, 114 c, and the like) may becollectively referred to as the pixel electrode 113 in some cases.

The pixel electrode 113 functions as a pixel electrode to which voltagedepending on each pixel is applied, and is formed of ITO (indium tinoxide), ZnO (zinc oxide), IZO formed by using a target in which ZnO of 2to 20 wt % is mixed to indium oxide, tin oxide (SnO), or the like. Partof the pixel electrode 113 is located above the electrode 108 and iselectrically connected to the electrode 108. Thus, the pixel electrode113 is electrically connected to the region 131 b of the thin filmtransistor 121 through the electrode 108.

Note that when the connection electrode 109 is not provided, the pixelelectrode 113 is directly connected to the region 131 b of the thin filmtransistor 121.

As shown in FIGS. 3 and 4, the pixel electrode 113 is generallyrectangular and includes a plurality of grooves 117 a, 117 b, 117 c, andthe like. Examples of the grooves 117 a, 117 b, 117 c, and the likeoften include grooves which are slit-shaped and in parallel to eachother.

In an example in FIG. 4, the grooves 117 a, 117 b, 117 c, and the likeare directed obliquely to the source wiring 107, and directions of thegrooves in the upper half of the pixel in the drawing and the grooves inthe lower half thereof are different from each other. By formation ofthe grooves 117 a, 117 b, 117 c, and the like, an electric field havinga component parallel to the substrate between the conductive film 115and the pixel electrode 113 is generated from each pixel electrode 114to the conductive film 115. Thus, orientation of liquid crystalsdescribed below can be controlled by controlling potentials of the pixelelectrodes 113 and 114.

In addition, as shown in FIG. 4, the directions of the groves 117 (117a, 117 b, 117 c, and the like) vary. Thus, a plurality of regions inwhich a direction toward which liquid crystal molecules move varies canbe provided. In other words, a multi-domain structure can be employed.With a multi-domain structure, a display defect can be prevented when animage is seen from a particular direction. Accordingly, a viewing anglecan be improved.

Note that a shape of the groove is not limited to those in thisembodiment mode. A shape of the groove includes a space in which aconductor is not formed, for example, a space between comb-shapedportions of a comb-shaped electrode.

When the thickness of the pixel electrode 113 and the thickness of theconductive film 115 are compared, the conductive film 115 is preferablythicker than the pixel electrode 113. More preferably, the conductivefilm 115 is 1.5 times or more as thick as the pixel electrode 113.Accordingly, resistance can be reduced.

As shown in FIG. 5, a first alignment film 112 and a liquid crystal 116are stacked over the second interlayer insulating film 111 and the pixelelectrode 113. As the liquid crystal 116, a ferroelectric liquid crystal(FLC), a bistable liquid crystal, a nematic liquid crystal, a smecticliquid crystal, a polymer dispersed liquid crystal, a liquid crystal tobe homogeneously aligned, a liquid crystal to be homeotropicallyaligned, or the like can be used. Alternatively, an element other than aliquid crystal, for example, an electrical image element may be used. Anopposite substrate 120 is provided over the liquid crystal 116 with asecond alignment film 123 and a color filter 122 interposedtherebetween. The substrate 101 and the opposite substrate 120 areprovided with polarizing plates 126 and 124, respectively.

Note that a retardation plate, a λ/4 plate, or the like is oftenprovided in addition to the polarizing plate.

Note that in the aforementioned structure, capacitance is formed byportions of the conductive film 115 and the pixel electrode 113, inwhich grooves are not formed, and by each insulating film interposedbetween the conductive film 115 and the pixel electrode 113. Storagecapacitance is increased by the formation of the capacitance.

Next, an example of a manufacturing method of a semiconductor device anda liquid crystal display device in the invention is described. First,the conductive film 115 having a light-transmitting property (e.g.,indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), tinoxide (SnO), or silicon (Si)) is formed over the entire surface of thesubstrate 101.

As shown in FIG. 25, the interlayer insulating film 106 may be removedin an opening portion. Alternatively, the gate insulating film 104 andthe base film 102 may be removed. That is, a semiconductor device inwhich the interlayer insulating film 106 is removed at an openingportion, a semiconductor device in which the insulating film 106 and thegate insulating film 104 are removed at an opening portion, or asemiconductor device in which the interlayer insulating film 106, thegate insulating film 104, and the base film 102 are removed at anopening portion can be formed. Accordingly, a distance d between theconductive film 115 and the pixel electrodes 114 (in FIG. 25, the pixelelectrodes 114 b, 114 c, and 114 d among the pixel electrodes 114 a to114 f) can be reduced; thus, an electric field can be easily controlled.

Next, the insulating film 102 is formed over each of the substrate 101and the conductive film 115. The insulating film 102 is preferablyformed thicker than the gate insulating film 104 described below. Then,a semiconductor film (e.g., a polycrystalline silicon film) is formedover the insulating film 102 and is selectively removed by etching withthe use of a resist. Thus, the island-shaped semiconductor film 103 isformed over the insulating film 102.

As the semiconductor film, an amorphous silicon film or anothernon-crystalline silicon film may be used as well as the polycrystallinesilicon film. Further, the invention is not limited to silicon, and acompound semiconductor such as ZnO, a-InGaZnO, SiGe, or GaAs may also beused.

Alternatively, a semiconductor substrate or an SOI (Silicon OnInsulator) substrate may be used as the substrate 101 to form theisland-shaped semiconductor film 103.

Next, the gate insulating film 104 is formed over the semiconductor film103 and the insulating film 102. The gate insulating film 104 is asilicon oxide film containing nitrogen or a silicon oxide film, forexample, and is formed by a plasma CVD method. Note that the gateinsulating film 104 may be formed of a silicon nitrogen film or astacked-layer film containing silicon nitride and silicon oxide. Next, aconductive film is formed over the gate insulating film 104 and isselectively removed by etching. Thus, the gate electrode 105 is formedover the gate insulating film 104 located over the semiconductor film103. Further, the gate wiring 105 and the wiring 119 are formed in thisstep.

Note that by provision of the wiring 119 as described above, a potentialof the conductive film 115 in each pixel can be stabilized.Alternatively, the wiring 119 is not formed in some cases. Further, thewiring 119 may be in another layer (e.g., the same layer as the sourcewiring 107, the same layer as the conductive film 115, or the same layeras the pixel electrode 113), or may be separately provided in aplurality of layers. In addition, although the wiring 119 is extended ina direction perpendicular to the source wiring 107 in the drawing, itmay be extended in the same direction as the source wiring 107.

The conductive film forming the gate electrode 105 and the wiring 119 ispreferably formed of one or a plurality of elements selected fromaluminum (Al), tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten(W), neodymium (Nd), chromium (Cr), nickel (Ni), platinum (Pt), gold(Au), silver (Ag), copper (Cu), magnesium (Mg), scandium (Sc), cobalt(Co), zinc (Zn), niobium (Nb), silicon (Si), phosphorus (P), boron (B),arsenic (As), gallium (Ga), indium (In), tin (Sn), and oxygen (O); acompound or an alloy material containing one or a plurality of theaforementioned elements (e.g., indium tin oxide (ITO), indium zinc oxide(IZO), indium tin oxide containing silicon oxide (ITSO), zinc oxide(ZnO), tin oxide (SnO), cadmium tin oxide (CTO), aluminum-neodymium(Al—Nd), magnesium-silver (Mg—Ag), or molybdenum-niobium (Mo—Nb)); asubstance in which these compounds are combined; or the like.Alternatively, the conductive film is preferably formed of a compound(silicide) of silicon and one or a plurality of the aforementionedelements (e.g., aluminum-silicon, molybdenum-silicon, ornickel-silicide), or a compound of nitrogen and one or a plurality ofthe aforementioned elements (e.g., titanium nitride, tantalum nitride,or molybdenum nitride).

Note that silicon (Si) may contain an n-type impurity (such asphosphorus) or a p-type impurity (such as boron). When silicon containsthe impurity, the conductivity is increased, and a function similar to ageneral conductor can be realized. Thus, such silicon can be utilizedeasily as a wiring, an electrode, or the like.

In addition, silicon with various levels of crystallinity, such assingle crystalline silicon, polycrystalline silicon, or microcrystallinesilicon can be used. Alternatively, silicon having no crystallinity,such as amorphous silicon can be used. By using single crystallinesilicon or polycrystalline silicon, resistance of a wiring, anelectrode, a conductive layer, a conductive film, a terminal, or thelike can be reduced. By using amorphous silicon or microcrystallinesilicon, a wiring or the like can be formed by a simple process.

Aluminum and silver have high conductivity, and thus can reduce a signaldelay. Further, since aluminum and silver can be easily etched, they canbe minutely processed.

Copper has high conductivity, and thus can reduce a signal delay. Whencopper is used, a stacked-layer structure is preferably employed toimprove adhesion.

Molybdenum and titanium are preferable since even if molybdenum ortitanium is in contact with an oxide semiconductor (e.g., ITO or IZO) orsilicon, molybdenum or titanium does not cause defects. Further,molybdenum and titanium are preferable since they are easily etched andhas high heat resistance.

Tungsten is preferable since it has an advantage such as high heatresistance.

Neodymium is also preferable since it has an advantage such as high heatresistance. In particular, an alloy of neodymium and aluminum ispreferable since heat resistance is increased and aluminum hardly causeshillocks.

Silicon is preferable since it can be formed at the same time as asemiconductor layer included in a transistor and has high heatresistance.

Since ITO, IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO),and cadmium tin oxide (CTO) have light-transmitting properties, they canbe used as a portion which transmits light. For example, they can beused for a pixel electrode or a common electrode.

IZO is preferable since it is easily etched and processed. In etchingIZO, a residue is hardly left. Thus, when IZO is used for a pixelelectrode, defects (such as short circuit or orientation disorder) of aliquid crystal element or a light-emitting element can be reduced.

The conductive film forming the gate electrode 105 and the wiring 119may have a single-layer structure or a multi-layer structure. Byemploying a single-layer structure, a manufacturing process of theconductive film forming the gate electrode 105 and the wiring 119 can besimplified, the number of days for a process can be reduced, and costcan be reduced. Alternatively, by employing a multi-layer structure, awiring, an electrode, and the like with high quality can be formed whilean advantage of each material is utilized and a disadvantage thereof isreduced. For example, when a low-resistant material (e.g., aluminum) isincluded in a multi-layer structure, reduction in resistance of a wiringcan be realized. Further, when a stacked-layer structure where a lowheat-resistant material is interposed between high heat-resistantmaterials is employed, heat resistance of a wiring, an electrode, andthe like can be increased, utilizing advantages of the lowheat-resistance material. For example, it is preferable to employ astacked-layer structure where a layer containing aluminum is interposedbetween layers containing molybdenum, titanium, neodymium, or the like.

When wirings, electrodes, or the like are in direct contact with eachother, they adversely affect each other in some cases. For example, onewiring or one electrode is mixed into a material of another wiring oranother electrode and changes its properties, and thus, an intendedfunction cannot be obtained in some cases. As another example, when ahigh-resistant portion is formed, a problem may occur so that it cannotbe normally formed. In such cases, a reactive material is preferablyinterposed by or covered with a non-reactive material in a stacked-layerstructure. For example, when ITO and aluminum are connected, titanium,molybdenum, or an alloy of neodymium is preferably interposed betweenITO and aluminum. As another example, when silicon and aluminum areconnected, titanium, molybdenum, or an alloy of neodymium is preferablyinterposed between silicon and aluminum.

Note that the term “wiring” indicates a portion including a conductor. Awiring may be provided linearly or may be short without being extendedlinearly. Therefore, an electrode is included in a wiring.

Next, an impurity is injected into the semiconductor film 103 by usingthe gate electrode 105 as a mask. Thus, the region 131 a which is one ofthe source region and the drain region, the region 131 b which is theother of the source region and the drain region, and the channelformation region 132 are formed in the semiconductor film 103. Note thatan n-type impurity element and a p-type impurity element may beseparately injected, or both the n-type impurity element and the p-typeimpurity element may be injected into a particular region. In the lattercase, the mount of one of the n-type impurity element and the p-typeimpurity element should be larger than the other. Note that in the step,a resist may be used for a mask.

At this time, an LDD region may be formed by changing the thickness or astacked-layer structure of the gate insulating film 104. In a portion inwhich the LDD region is to be formed, the gate insulating film 104 maybe thickened or the number of layers may be increased. Thus, the amountof impurity to be injected is reduced, and an LDD region can be easilyformed.

Note that the impurity may be injected into the semiconductor film 103before the gate electrode 105 is formed, for example, before or afterthe gate insulating film 104 is formed. In this case, a resist is used amask. Accordingly, capacitance can be formed between the electrode inthe same layer as the gate and the semiconductor film into which theimpurity is injected. The gate insulating film is provided between theelectrode in the same layer as the gate and the semiconductor film intowhich the impurity is injected, so that capacitance with thin thicknessand large capacity can be formed.

Next, the first interlayer insulating film 106 is formed, and further, acontact hole is formed. Then, a conductive film (e.g., a metal film) isformed over the first interlayer insulating film 106 and is selectivelyremoved by etching with the use of a mask. Thus, the source wiring 107,the electrode 108, and the connection electrode 109 are formed.

Next, the second interlayer insulating film 111 is formed, and further,a contact hole is formed. Then, a conductive film having alight-transmitting property (e.g., indium tin oxide (ITO), indium zincoxide (IZO), zinc oxide (ZnO), tin oxide (SnO), or silicon (Si)) isformed over the second interlayer insulating film 111, and isselectively removed by etching with the use of a resist. Thus, the pixelelectrode 113 is formed.

When the contact hole filled with part of the electrode 108 and thecontact hole filled with part of the pixel electrode 113 are located atthe same position, layout can be efficiently performed because thecontact holes are located at one place. Thus, an aperture ratio of thepixel can be improved.

On the other hand, the contact hole filled with part of the electrode108 and the contact hole filled with part of the pixel electrode 113 maybe located at different positions. Thus, when portions of the electrode108 and the pixel electrode 113, which are located above the contactholes, are depressed, the depressed portions do not overlap with eachother. Thus, the deeply depressed portion is not formed at the pixelelectrode 113, and generation of a defect of the resist can besuppressed. Thereafter, the resist is removed.

Next, the first alignment film 112 is formed, and the liquid crystal 116is sealed between the first alignment film 112 and the oppositesubstrate 120 on which the color filter 122 and the second alignmentfilm 123 are formed. Thereafter, sides of the opposite substrate 120 andthe substrate 101, which are not in contact with the liquid crystal 116,are provided with the polarizing plates 126 and 124, a retardation plate(not shown), an optical film (not shown) such as a λ/4 plate, an opticalfilm such as a diffusing plate or a prism sheet, or the like. Further, abacklight or a front light is provided. As the backlight, a direct typebacklight or a sidelight type backlight can be used. As a light source,a cold cathode tube or an LED (a light-emitting diode) can be used. Asthe LED, a white LED or a combination of LEDs of respective colors(e.g., white, red, blue, green, cyan, magenta, and/or yellow) may beused. By using the LED, color purity can be improved because the LED hasa sharp peak of light wavelength. In the case of a sidelight typebacklight, a light guide plate is provided to realize a uniform surfacelight source. The liquid crystal display device is formed in such amanner.

Note that a liquid crystal display device may only refer to a substrate,an opposite substrate, and a liquid crystal interposed therebetween.Alternatively, the liquid crystal display device may further include anoptical film such as a polarizing plate or a retardation plate. Furtheralternatively, the liquid crystal display device may also include adiffusing plate, a prism sheet, a light source (such as a cold cathodetube or an LED), a light-guide plate, and the like.

In this embodiment mode, a so-called top-gate thin film transistor inwhich a gate electrode is located above a channel region is described;however, the present invention is not particularly limited thereto. Aso-called bottom-gate thin film transistor in which a gate electrodelocated below a channel region, or a transistor having a structure inwhich gate electrodes are arranged above and below a channel region maybe formed.

In this embodiment mode, a so-called single-gate thin film transistor inwhich one gate electrode is formed is described; however, a so-calledmulti-gate thin film transistor in which two or more gate electrodes areformed may be formed.

A liquid crystal display device may be a transmissive liquid crystaldisplay device or a reflective liquid crystal display device. Forexample, the conductive film 115 is formed of a light-transmitting film(e.g., an indium tin oxide (ITO) film, an indium zinc oxide (IZO) film,a zinc oxide (ZnO) film, or a polycrystalline silicon film or anamorphous silicon film into which an impurity is introduced), and thepixel electrode 113 is formed of a reflective conductive film, forexample, a metal film. Thus, a reflective liquid crystal display devicecan be realized. In addition, the pixel electrode 113 is formed of alight-transmitting film, and part of the conductive film 115 is formedof a reflective conductive film, for example, a metal film, whereas theother part of the conductive film 115 is formed of a light-transmittingfilm; thus, a transflective liquid crystal display device can berealized.

In a reflective liquid crystal display device, when a reflectiveconductive film, for example, a metal film is used as the conductivefilm 115, the conductive film 115 can have a function as a reflectingplate. A reflective conductive film can be used as one or both of thepixel electrode 113 and the conductive film 115. Further, an insulatingfilm (e.g., a silicon oxide film) may be provided between the substrate101 and the conductive film 115, and a metal film as a reflecting filmmay be formed in the insulating film. A reflecting sheet (e.g., analuminum film) as a reflecting film may be formed on an external surfaceof the substrate 101. Note that the contents described here can besimilarly applied to embodiment modes described below.

According to this embodiment mode, a liquid crystal display device witha wide viewing angle and lower manufacturing cost than a conventionalliquid crystal display device can be provided.

In this embodiment mode, since a conductive film is formed over anentire surface of a substrate, an impurity from the substrate can beprevented from being mixed into an active layer. Thus, a semiconductordevice with high reliability can be obtained.

In this embodiment mode, since a semiconductor device including atop-gate thin film transistor is formed, a potential of a back gate isstabilized; thus, a semiconductor device with high reliability can beobtained.

Embodiment Mode 2

In this embodiment mode, an example in which a bottom-gate TFT is formedas a switching element in a pixel portion is described with reference toFIG. 2.

A conductive film 202, a base film 203, a gate electrode 204, a gateinsulating film 213, an island-shaped semiconductor film 206 to be anactive layer, a region 208 a which is one of a source region and a drainregion, a region 208 b which is the other of the source region and thedrain region, an electrode 207 a which is one of a source electrode anda drain electrode, an electrode 207 b which is the other of the sourceelectrode and the drain electrode, and pixel electrodes 209 and 214 (214a, 214 b, 214 c, and the like) are formed over a substrate 201. A TFT212 includes the gate electrode 204, the gate insulating film 213, theisland-shaped semiconductor film 206, and the regions 208 a and 208 b.

A horizontal electric field 225 is generated between the pixelelectrodes 214 and the conductive film 202. Liquid crystal molecules aredriven by the horizontal electric field 225.

An electrode 205 formed of the same material and in the same step as thegate electrode 204 is formed over the base film 203. An electrode 211formed of the same material and in the same step as the pixel electrode209 is formed over an insulating film 210. The electrode 211 iselectrically connected to the conductive film 202 and the electrode 205through contact holes formed in the base film 203, the gate insulatingfilm 213, and the insulating film 210.

As the substrate 201, a material similar to that of the substrate 101may be used.

As the conductive film 202, a conductive film similar to the conductivefilm 115 described in Embodiment Mode 1 may be used.

The base film 203 may be formed of a material similar to that of thebase film 102.

The gate electrode 204 and the electrode 205 may be formed of a materialand in a step similar to those of the gate electrode 105. The gateinsulating film 213 is formed of a material similar to that of the gateinsulating film 104 or the interlayer insulating film 106 over an entiresurface of the substrate 201.

In this embodiment mode, a so-called single-gate thin film transistor inwhich one gate electrode is formed is described; however, a so-calledmulti-gate thin film transistor in which two or more gate electrodes areformed may be formed.

The island-shaped semiconductor film 206, which is the active layer, isformed of a material similar to that of the island-shaped semiconductorfilm 103, and preferably, an amorphous semiconductor film or amicrocrystalline semiconductor film (a semi-amorphous semiconductorfilm). In that case, after an intrinsic semiconductor film (theisland-shaped semiconductor film 206) is formed, a semiconductor filmcontaining an impurity imparting one conductivity type is formed. As theimpurity imparting one conductivity type, phosphorus (P) or arsenic (As)may be used as an impurity imparting n-type conductivity, and boron (B)may be used as an impurity imparting p-type conductivity. A bottom-gateTFT in this embodiment mode employs a channel etch type; thus, after anisland-shaped semiconductor film, a source electrode, and a drainelectrode are formed, part of a channel formation region is needed to beetched.

Next, a conductive film is formed over the gate insulating film 213 andthe island-shaped semiconductor film 206, and the electrodes 207 a and207 b are formed by etching. Thereafter, by using the electrodes 207 aand 207 b as masks, part of the semiconductor film containing theimpurity imparting one conductivity type is etched to form the regions208 a and 208 b.

The insulating film 210 is formed over the island-shaped semiconductorfilm 206, the regions 208 a and 208 b, and the electrodes 207 a and 207b. The insulating film 210 may be formed of a material and in a stepsimilar to those of the interlayer insulating film 106 or the interlayerinsulating film 111. Note that when an organic material is not used forthe insulating film 210, a distance d between the pixel electrode 214and the conductive film 202 can be reduced; thus, an electric field canbe easily controlled.

The pixel electrodes 209 and 214 (214 a, 214 b, 214 c, and the like) andthe electrode 211 are formed over the insulating film 210. The pixelelectrodes 209 and 214 are formed in such a manner that grooves areformed in a conductive film, similar to the pixel electrodes 113 and114.

The electrode 211 is electrically connected to the electrode 205 througha contact hole formed in the gate insulating film 213 and the insulatingfilm 210, and is also electrically connected to the conductive film 202through a contact hole formed in the base film 203, the gate insulatingfilm 213, and the insulating film 210.

An alignment film 215 is formed over the pixel electrodes 209 and 214,and the electrode 211. The alignment film 215 may be formed of amaterial similar to that of the alignment film 112.

A color filter 222 and an alignment film 223 are formed over an oppositesubstrate 221. The opposite substrate 221, the color filter 222, and thealignment film 223 may be formed of a material similar to that of theopposite substrate 120, the color filter 122, and the alignment film123, respectively.

The alignment film 223 over the opposite substrate 221 and the alignmentfilm 215 over the substrate 201 face each other, and a liquid crystal216 is injected into a space.

Thereafter, sides of the opposite substrate 221 and the substrate 201,which are not in contact with the liquid crystal 216, are provided withpolarizing plates 224 and 217, a retardation plate (not shown), anoptical film (not shown) such as a λ/4 plate, an optical film such as adiffusing plate or a prism sheet, or the like. Further, a backlight or afront light is provided. As the backlight, a direct type backlight or asidelight type backlight can be used. As a light source, a cold cathodetube or an LED (a light-emitting diode) can be used. As the LED, a whiteLED or a combination of LEDs of respective colors (e.g., white, red,blue, green, cyan, magenta, and/or yellow) may be used. By using theLED, color purity can be improved because the LED has a sharp peak oflight wavelength. In the case of a sidelight type backlight, a lightguide plate is provided to realize a uniform surface light source. Theliquid crystal display device is formed in such a manner.

FIG. 26 shows an example where an active layer of the TFT 212 is formedof a crystalline semiconductor film. In FIG. 26, the portions same asthose in FIG. 2 are denoted by the same reference numerals. In FIG. 26,the TFT 212 includes an island-shaped crystalline semiconductor film 253as an active layer. The island-shaped crystalline semiconductor film 253includes a channel formation region 256, a region 258 a which is one ofa source region and a drain region, and a region 258 b which is theother of the source region and the drain region.

Instead of the electrode 211 in FIG. 2, an electrode 251 formed of thesame material and in the same step as the electrode 207 a which is oneof the source electrode and the drain electrode and the electrode 207 bwhich is the other of the source electrode and the drain electrode isused.

Note that since this embodiment mode is similar to Embodiment Mode 1,except that the TFT 121 in Embodiment Mode 1 is replaced with thebottom-gate TFT 212 in this embodiment mode, materials and manufacturingsteps of the other structures can refer to those described in EmbodimentMode 1.

According to this embodiment mode, a liquid crystal display device witha wide viewing angle and lower manufacturing cost than a conventionalliquid crystal display device can be provided.

In the invention, since a conductive film is formed over an entiresurface of a substrate, an impurity from the substrate can be preventedfrom being mixed into an active layer. Thus, a semiconductor device withhigh reliability can be obtained.

Embodiment Mode 3

In this embodiment mode, FIG. 6 shows an example where the electrode 108in Embodiment Mode 1 is not formed and the pixel electrode 113 is formedto be directly connected to the region 131 b. The reference numerals inEmbodiment Mode 1 are used for reference numerals in FIG. 6. Materialsand manufacturing steps of the other structures can refer to thosedescribed in Embodiment Mode 1. This embodiment mode has an advantage ofimprovement in aperture ratio because the electrode 108 is not formed.

A bottom-gate TFT described in Embodiment Mode 2 may be used whenneeded.

According to this embodiment mode, a liquid crystal display device witha wide viewing angle and lower manufacturing cost than a conventionalliquid crystal display device can be provided.

In the invention, since a conductive film is formed over an entiresurface of a substrate, an impurity from the substrate can be preventedfrom being mixed into an active layer. Thus, a semiconductor device withhigh reliability can be obtained.

In the invention, when a semiconductor device including a top-gate thinfilm transistor is formed, a potential of a back gate is stabilized;thus, a semiconductor device with high reliability can be obtained.

Embodiment Mode 4

This embodiment mode is described with reference to FIGS. 10 to 13. Thereference numerals in Embodiment Mode 1 are used for reference numeralsin FIGS. 10 to 13. Materials and manufacturing steps of the otherstructures may refer to those described in Embodiment Mode 1.

A bottom-gate TFT described in Embodiment Mode 2 may be used whenneeded.

Further, a structure where a pixel electrode is directly connected to anactive layer, which is described in Embodiment Mode 3, may be used whenneeded.

In FIG. 10, an electrode 141 formed of a material and in a step similarto those of the pixel electrode 113 is used instead of the connectionelectrode 109 in FIG. 6. The wiring 119 and the conductive film 115 areelectrically connected through the electrode 141.

FIG. 11 is a top plan view of FIG. 10. In FIG. 11, the portions same asthose in FIGS. 4 and 10 are denoted by the same reference numerals. FIG.10 is a cross-sectional view of C-C′ and D-D′ in FIG. 11.

In FIG. 12, the electrode 141 formed of a material and in a step similarto those of the pixel electrode 113; and an electrode 142 formed of amaterial and in a step similar to those of the electrodes 107 and 108are used instead of the connection electrode 109 in FIG. 6. The wiring119 and the conductive film 115 are electrically connected through theelectrodes 141 and 142.

FIG. 13 is a top plan view of FIG. 12. In FIG. 13, the portions same asthose in FIGS. 4 and 12 are denoted by the same reference numerals. FIG.12 is a cross-sectional view of C-C′ and D-D′ in FIG. 13.

According to this embodiment mode, a liquid crystal display device witha wide viewing angle and lower manufacturing cost than a conventionalliquid crystal display device can be provided.

In the invention, since a conductive film is formed over an entiresurface of a substrate, an impurity from the substrate can be preventedfrom being mixed into an active layer. Thus, a semiconductor device withhigh reliability can be obtained.

In the invention, when a semiconductor device including a top-gate thinfilm transistor is formed, a potential of a back gate is stabilized;thus, a semiconductor device with high reliability can be obtained.

Embodiment Mode 5

In this embodiment mode, FIGS. 7, 8A to 8D, and 9A to 9D show examplesof pixel electrodes with various shapes. The reference numerals inEmbodiment Mode 1 are used for reference numerals in FIGS. 7, 8A to 8D,and 9A to 9D. Materials and manufacturing steps of the other structuresmay refer to those described in Embodiment Mode 1.

A bottom-gate TFT described in Embodiment Mode 2 may be used whenneeded.

Further, a structure where a pixel electrode is directly connected to anactive layer, which is described in Embodiment Mode 3, may be used whenneeded.

Moreover, a connection structure of the conductive film 115 and thewiring 119, which is described in Embodiment Mode 4, may be used whenneeded.

FIG. 7 shows the pixel electrode 113 with a comb shape. Across-sectional view of A-A′ and B-B′ is same as FIG. 3. FIGS. 8A to 8Dand 9A to 9D only show the pixel electrode 113 and the conductive film115 so that each drawing is understandable.

In FIG. 8A, a plurality of slit-shaped openings are formed in the pixelelectrode 113. The slit-shaped openings are oblique to the sourcewiring. The slit-shaped openings in the upper half of the pixelelectrode 113 and the slit-shaped openings in the lower half thereofhave different angles with respect to the center line of the pixelelectrode 113. The slit-shaped openings in the upper half of the pixelelectrode 113 and the slit-shaped openings in the lower half thereof maybe line-symmetrical with respect to the central line.

In FIG. 8B, the pixel electrode 113 has a shape in which a plurality ofelectrodes each having a shape along a circumference of a circle, radiusof which is different from each other, are arranged in a concentricpattern and are connected to each other. Each space between theelectrodes functions as an opening.

In FIG. 8C, the pixel electrode 113 is arranged so that two comb-shapedelectrodes are arranged to face opposite directions to each other andcomb-shaped portions are alternately arranged. Each space betweencomb-shaped portions functions as an opening.

In FIG. 8D, the pixel electrode 113 is comb-shaped. Each space betweencomb-shaped portions functions as an opening.

In FIG. 9A, the pixel electrode 113 is formed in stripes in an obliquedirection. Each space between stripe portions functions as an opening.

In FIG. 9B, a plurality of rectangular opening portions are formed inthe pixel electrode 113.

In FIG. 9C, an opening portion with a shape in which two sides of anelongated rectangular, which face each other, are wave-shaped, is formedin the pixel electrode 113.

In FIG. 9D, an opening portion with an elongated rectangular shape isformed in the pixel electrode 113.

According to the invention, a liquid crystal display device with a wideviewing angle and lower manufacturing cost than a conventional liquidcrystal display device can be provided.

In the invention, since a conductive film is formed over an entiresurface of a substrate, an impurity from the substrate can be preventedfrom being mixed into an active layer. Thus, a semiconductor device withhigh reliability can be obtained.

In the invention, when a semiconductor device including a top-gate thinfilm transistor is formed, a potential of a back gate is stabilized;thus, a semiconductor device with high reliability can be obtained.

Embodiment Mode 6

In this embodiment mode, examples where a color filter is provided at aplace different from that in Embodiment Mode 1 are described withreference to FIGS. 22, 23A, 23B, and 24.

FIG. 22 is a cross-sectional view for describing a structure of a pixelportion in an FFS mode liquid crystal display device according to thisembodiment mode. The pixel portion in the liquid crystal display deviceaccording to this embodiment mode has a structure similar to that of theliquid crystal display device in Embodiment Mode 1, except that a colorfilter is not provided on the opposite substrate 120 side and colorfilters 241 (a red color filter 241R, a blue color filter 241B, and agreen color filter 241G) are provided instead of the interlayerinsulating film 106.

Accordingly, the contents described in other embodiment modes exceptEmbodiment Mode 1 can also be applied to this embodiment mode.Hereinafter, the portions similar to those in Embodiment Mode 1 aredenoted by the same reference numerals, and description thereof isomitted.

Note that an insulating film formed of an inorganic material may beprovided between the color filter 241 and the gate electrode 105. Theinorganic material is formed of an insulating material containing oxygenor nitrogen, such as silicon oxide, silicon nitride, silicon oxidecontaining nitrogen, or silicon nitride containing oxygen. It ispreferable to use a material containing a large amount of nitrogen inorder to prevent penetration of an impurity. Further, a planarizationfilm may be formed over the color filter 241.

Note that colors of the color filters 241 may be colors other than red,blue, and green, and may be more than three colors, for example, fourcolors or six colors. For example, yellow, cyan, magenta, or white maybe added. Further, a black matrix may be provided as well as a colorfilter. The black matrix may be formed using a resin material, a metalfilm, or carbon black.

By providing the color filters 241 over the substrate 101 in such amanner, it is not necessary to perform precise alignment with theopposite substrate 120; thus, a liquid crystal display device can beeasily formed, cost is reduced, and manufacturing yield is improved.

A method for manufacturing the liquid crystal display device accordingto this embodiment mode is similar to the method for manufacturing theliquid crystal display device in Embodiment Mode 1, except that a stepof forming the color filters 241 (241R, 241G, and 241B) is added insteadof the step of forming the interlayer insulating film 106.

The color filters 241R, 241G, and 241B are formed by repeating thefollowing steps three times: a step of forming a color filter layer, astep of forming a resist over the color filter layer, and a step ofselectively dry-etching the color filter layer with use of the resist asa mask.

Alternatively, the color filters may be formed by using a photosensitivematerial, pigment, or the like without using a resist. Note that a spaceis generated between the color filter layers, and the interlayerinsulating film 111 is embedded in this space. Alternatively, aninorganic material or an organic material may be further stacked.Further alternatively, a black matrix or the like may be stacked. Thecolor filters 241R, 241G, and 241B, or the black matrix can also beformed by using a droplet discharging method (e.g., an ink-jet method).

Thus, the number of manufacturing steps of the liquid crystal displaydevice can be reduced. Since the color filters are provided on thesubstrate 101 side, decrease in aperture ratio can be suppressed evenwhen misalignment with the opposite substrate 120 is caused, as comparedwith the case where the color filters are provided on the oppositesubstrate 120. That is, a margin for misalignment of the oppositesubstrate 120 increases.

FIG. 23A is a plan view of the liquid crystal display device shown inFIG. 22. As shown in FIG. 23A, in the liquid crystal display device ofthis embodiment mode, a source line driver circuit 152 and a gate linedriver circuit 154 which are peripheral driver circuits are providedaround a pixel portion 150.

The red color filter 241R may be provided over each of the source linedriver circuit 152 and the gate line driver circuit 154. Provision ofthe color filter 241R prevents light deterioration of an active layer ofeach thin film transistor included in the source line driver circuit 152and the gate line driver circuit 154 and realizes planarization.

FIG. 23B is an enlarged view of a part (three rows x three columns) ofthe pixel portion 150 in FIG. 23A. In the pixel portion 150, the redcolor filter 241R, the blue color filter 241B, and the green colorfilter 241G are alternately arranged in stripes. Further, the red colorfilter 241R is provided over a thin film transistor included in eachpixel.

Since a source wiring (not shown) and a gate wiring (not shown) arearranged to overlap with the space between the color filters, lightleakage is suppressed.

Since the color filter 241 functions as a black matrix in this manner, astep of forming a black matrix, which is conventionally required, can beomitted.

As described above, according to this embodiment mode, advantageouseffects similar to other embodiment modes can be obtained. Further,since the color filters 241R, 241G, and 241B are provided instead of theinterlayer insulating film 106, the number of manufacturing steps of theliquid crystal display device can be reduced. Moreover, reduction inaperture ratio can be suppressed even when misalignment with theopposite substrate 120 is caused, as compared with the case where thecolor filter is provided on the opposite substrate 120. That is, amargin for misalignment of the opposite substrate 120 is increased.

In addition, a black matrix may be provided in addition to a colorfilter.

Note that in the FFS mode liquid crystal display devices shown in otherembodiment modes, the color filters 241 (241R, 241G, and 241B) may beprovided instead of the interlayer insulating film 106 or instead of thesecond interlayer insulating film 111 (see FIG. 24), similar to thisembodiment mode. In this case also, advantageous effects similar tothose of this embodiment mode can be obtained.

Embodiment Mode 7

In this embodiment mode, a structure and a manufacturing method of atransistor are described.

FIGS. 29A to 29G show a structure and a manufacturing method of atransistor. FIG. 29A shows a structure example of a transistor. FIGS.29B to 29G show an example of a manufacturing method of the transistor.

Note that the structure and the manufacturing method of a transistor arenot limited to those shown in FIGS. 29A to 29G, and various structuresand manufacturing methods can be employed.

First, a structure example of a transistor is described with referenceto FIG. 29A. FIG. 29A is a cross-sectional view of a plurality oftransistors each having a different structure. Here, in FIG. 29A, theplurality of transistors each having a different structure arejuxtaposed, which is for describing structures of the transistors.Therefore, the transistors are not needed to be actually juxtaposed asshown in FIG. 29A and can be separately formed as needed.

Next, characteristics of each layer forming the transistor aredescribed.

A substrate 7011 can be a glass substrate using barium borosilicateglass, aluminoborosilicate glass, or the like, a quartz substrate, aceramic substrate, a metal substrate containing stainless steel, or thelike. In addition, a substrate formed of plastics typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN), orpolyethersulfone (PES), or a substrate formed of a flexible syntheticresin such as acrylic can also be used. By using a flexible substrate, asemiconductor device capable of being bent can be formed. A flexiblesubstrate has no strict limitations on an area or a shape of thesubstrate. Therefore, for example, when a substrate having a rectangularshape, each side of which is 1 meter or more, is used as the substrate7011, productivity can be significantly improved. Such an advantage ishighly favorable as compared with the case where a circular siliconsubstrate is used.

An insulating film 7012 functions as a base film and is provided toprevent alkali metal such as Na or alkaline earth metal from thesubstrate 7011 from adversely affecting characteristics of asemiconductor element. The insulating film 7012 can have a single-layerstructure or a stacked-layer structure of an insulating film containingoxygen or nitrogen, such as silicon oxide (SiOx), silicon nitride(SiNx), silicon oxynitride (SiOxNy) (x>y), or silicon nitride oxide(SiNxOy) (x>y). For example, when the insulating film 7012 is providedto have a two-layer structure, it is preferable that a silicon nitrideoxide film be used as a first insulating film and a silicon oxynitridefilm be used as a second insulating film. As another example, when theinsulating film 7012 is provided to have a three-layer structure, it ispreferable that a silicon oxynitride film be used as a first insulatingfilm, a silicon nitride oxide film be used as a second insulating film,and a silicon oxynitride film be used as a third insulating film.

Semiconductor layers 7013, 7014, and 7015 can be formed using anamorphous semiconductor, a microcrystalline semiconductor, or asemi-amorphous semiconductor (SAS). Alternatively, a polycrystallinesemiconductor layer may be used. SAS is a semiconductor having anintermediate structure between amorphous and crystalline (includingsingle crystal and polycrystalline) structures and having a third statewhich is stable in free energy. Moreover, SAS includes a crystallineregion with a short-range order and lattice distortion. A crystallineregion of 0.5 to 20 nm can be observed at least in part of a film. Whensilicon is contained as a main component, Raman spectrum shifts to awave number side lower than 520 cm⁻¹. The diffraction peaks of (111) and(220), which are thought to be derived from a silicon crystallinelattice, are observed by X-ray diffraction. SAS contains hydrogen orhalogen of at least 1 atomic % or more to compensate dangling bonds. SASis formed by glow discharge decomposition (plasma CVD) of a materialgas. As the material gas, SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, orthe like can be used. Further, GeF₄ may be mixed. Alternatively, thematerial gas may be diluted with H₂, or H₂ and one or more kinds of raregas elements selected from He, Ar, Kr, and Ne. A dilution ratio is inthe range of 2 to 1000 times. Pressure is in the range of approximately0.1 to 133 Pa, and a power supply frequency is 1 to 120 MHz, preferably13 to 60 MHz. A substrate heating temperature may be 300° C. or lower. Aconcentration of impurities in atmospheric components such as oxygen,nitrogen, and carbon is preferably 1×10²⁰ cm⁻¹ or less as impurityelements in the film. In particular, an oxygen concentration is5×10¹⁹/cm³ or less, preferably 1×10¹⁹/cm³ or less. Here, an amorphoussemiconductor layer is formed using a material containing silicon (Si)as its main component (e.g., Si_(x)Ge_(1-x)) by a sputtering method, anLPCVD method, a plasma CVD method, or the like. Then, the amorphoussemiconductor layer is crystallized by a crystallization method such asa laser crystallization method, a thermal crystallization method usingRTA or an annealing furnace, or a thermal crystallization method using ametal element which promotes crystallization.

An insulating film 7016 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx), siliconoxynitride (SiOxNy) (x>y), or silicon nitride oxide (SiNxOy) (x>y).

A gate electrode 7017 can have a single-layer structure of a conductivefilm or a stacked-layer structure of two or three conductive films. As amaterial for the gate electrode 7017, a conductive film can be used. Forexample, a single film of an element such as tantalum (Ta), titanium(Ti), molybdenum (Mo), tungsten (W), chromium (Cr), silicon (Si), or thelike; a nitride film containing the aforementioned element (typically, atantalum nitride film, a tungsten nitride film, or a titanium nitridefilm); an alloy film in which the aforementioned elements are combined(typically, a Mo—W alloy or a Mo—Ta alloy); a silicide film containingthe aforementioned element (typically, a tungsten silicide film or atitanium silicide film); and the like can be used. Note that theaforementioned single film, nitride film, alloy film, silicide film, andthe like can have a single-layer structure or a stacked-layer structure.

An insulating film 7018 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx), siliconoxynitride (SiOxNy) (x>y), or silicon nitride oxide (SiNxOy) (x>y); or afilm containing carbon, such as a DLC (Diamond-Like Carbon), by asputtering method, a plasma CVD method, or the like.

An insulating film 7019 can have a single-layer structure or astacked-layer structure of a siloxane resin; an insulating filmcontaining oxygen or nitrogen, such as silicon oxide (SiOx), siliconnitride (SiNx), silicon oxynitride (SiOxNy) (x>y), or silicon nitrideoxide (SiNxOy) (x>y); a film containing carbon, such as a DLC(Diamond-Like Carbon); or an organic material such as epoxy, polyimide,polyamide, polyvinyl phenol, benzocyclobutene, or acrylic. Note that asiloxane resin corresponds to a resin having Si—O—Si bonds. Siloxaneincludes a skeleton structure of a bond of silicon (Si) and oxygen (O).As a substituent, an organic group containing at least hydrogen (such asan alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluorogroup, or a fluoro group and an organic group containing at leasthydrogen can be used as a substituent. Note that the insulating film7019 can be provided to cover the gate electrode 7017 directly withoutprovision of the insulating film 7018.

As a conductive film 7023, a single film of an element such as Al, Ni,C, W, Mo, Ti, Pt, Cu, Ta, Au, Mn, or the like, a nitride film containingthe aforementioned element, an alloy film in which the aforementionedelements are combined, a silicide film containing the aforementionedelement, or the like can be used. For example, as an alloy containing aplurality of the aforementioned elements, an Al alloy containing C andTi, an Al alloy containing Ni, an Al alloy containing C and Ni, an Alalloy containing C and Mn, or the like can be used. When the conductivefilm has a stacked-layer structure, a structure can be such that Al isinterposed between Mo, Ti, or the like; thus, resistance of Al to heatand chemical reaction can be improved.

Next, characteristics of each structure are described with reference tothe cross-sectional view of the plurality of transistors each having adifferent structure in FIG. 29A.

A transistor 7001 is a single drain transistor. Since it can be formedby a simple method, it is advantageous in low manufacturing cost andhigh yield. Note that a tapered angle is 45° or more and less than 95°,and preferably, 60° or more and less than 95°. The tapered angel may beless than 45°. Here, the semiconductor layers 7013 and 7015 havedifferent concentrations of impurities, and the semiconductor layer 7013is used as a channel region and the semiconductor layers 7015 are usedas a source region and a drain region. By controlling the concentrationof impurities in this manner, resistivity of the semiconductor layer canbe controlled. Further, an electrical connection state of thesemiconductor layer and the conductive film 7023 can be closer to ohmiccontact. Note that as a method of separately forming the semiconductorlayers each having different amount of impurities, a method whereimpurities are doped in the semiconductor layer using the gate electrode7017 as a mask can be used.

In a transistor 7002, the gate electrode 7017 is tapered at an angle ofat least certain degrees. Since it can be formed by a simple method, itis advantageous in low manufacturing cost and high yield. Here, thesemiconductor layers 7013, 7014, and 7015 have different concentrationsof impurities. The semiconductor layer 7013 is used as a channel region,the semiconductor layers 7014 as lightly doped drain (LDD) regions, andthe semiconductor layers 7015 as a source region and a drain region. Bycontrolling the amount of impurities in this manner, resistivity of thesemiconductor layer can be controlled. Further, an electrical connectionstate of the semiconductor layer and the conductive film 7023 can becloser to ohmic contact. Moreover, since the transistor includes the LDDregions, high electric field is hardly applied inside the transistor, sothat deterioration of the element due to hot carriers can be suppressed.Note that as a method of separately forming the semiconductor layershaving different amount of impurities, a method where impurities aredoped in the semiconductor layer using the gate electrode 7017 as a maskcan be used. In the transistor 7002, since the gate electrode 7017 istapered at an angle of at least certain degrees, gradient of theconcentration of impurities doped in the semiconductor layer through thegate electrode 7017 can be provided, and the LDD region can be easilyformed. Note that the tapered angle is 45° or more and less than 95°,and preferably, 60° or more and less than 95°. Alternatively, thetapered angel can be less than 45°.

A transistor 7003 has a structure where the gate electrode 7017 isformed of at least two layers and a lower gate electrode is longer thanan upper gate electrode. In this specification, a shape of the lower andupper gate electrodes is called a hat shape. When the gate electrode7017 has a hat shape, an LDD region can be formed without addition of aphotomask. Note that a structure where the LDD region overlaps with thegate electrode 7017, like the transistor 7003, is particularly called aGOLD (Gate Overlapped LDD) structure. As a method of forming the gateelectrode 7017 with a hat shape, the following method may be used.

First, when the gate electrode 7017 is etched, the lower and upper gateelectrodes are etched by dry etching so that side surfaces thereof areinclined (tapered). Then, an inclination of the upper gate electrode isprocessed to be almost perpendicular by anisotropic etching. Thus, thegate electrode a cross section of which is a hat shape is formed. Afterthat, impurity elements are doped twice, so that the semiconductor layer7013 used as the channel region, the semiconductor layers 7014 used asthe LDD regions, and the semiconductor layers 7015 used as a sourceelectrode and a drain electrode are formed.

Note that part of the LDD region, which overlaps with the gate electrode7017, is referred to as an Lov region, and part of the LDD region, whichdoes not overlap with the gate electrode 7017, is referred to as an Loffregion. The Loff region is highly effective in suppressing anoff-current value, whereas it is not very effective in preventingdeterioration in an on-current value due to hot carriers by relieving anelectric field in the vicinity of the drain. On the other hand, the Lovregion is highly effective in preventing deterioration in the on-currentvalue by relieving the electric field in the vicinity of the drain,whereas it is not very effective in suppressing the off-current value.Thus, it is preferable to form a transistor having a structureappropriate for characteristics of each of the various circuits. Forexample, when a semiconductor device is used for a display device, atransistor having an Loff region is preferably used as a pixeltransistor in order to suppress the off-current value. On the otherhand, as a transistor in a peripheral circuit, a transistor having anLov region is preferably used in order to prevent deterioration in theon-current value by relieving the electric field in the vicinity of thedrain.

A transistor 7004 includes a sidewall 7021 in contact with the sidesurface of the gate electrode 7017. When the transistor includes thesidewall 7021, a region overlapping with the sidewall 7021 can be madeto be an LDD region.

In a transistor 7005, an LDD (Loft) region is formed by doping in thesemiconductor layer with the use of a mask 7022. Thus, the LDD regioncan surely be formed, and an off-current value of the transistor can bereduced.

In a transistor 7006, an LDD (Lov) region is formed by doping in thesemiconductor layer with the use of a mask. Thus, the LDD region cansurely be formed, and deterioration in an on-current value can beprevented by relieving the electric field in the vicinity of the drainof the transistor.

Next, an example of a method for manufacturing a transistor is describedwith reference to FIGS. 29B to 29G.

Note that a structure and a manufacturing method of a transistor are notlimited to those in FIGS. 29A to 29G, and various structures andmanufacturing methods can be used.

In this embodiment mode, surfaces of the substrate 7011, the insulatingfilm 7012, the semiconductor layers 7013, 7014, and 7015, the insulatingfilm 7016, the insulating film 7018, or the insulating film 7019 areoxidized or nitrided by plasma treatment, so that the semiconductorlayer or the insulating film can be oxidized or nitrided. By oxidizingor nitriding the semiconductor layer or the insulating film by plasmatreatment in such a manner, a surface of the semiconductor layer or theinsulating film is modified, and the insulating film can be formed to bedenser than an insulating film formed by a CVD method or a sputteringmethod. Thus, a defect such as a pinhole can be suppressed, andcharacteristics and the like of a semiconductor device can be improved.Note that an insulating film 7024 formed by plasma treatment is referredto as a plasma-treated insulating film.

Silicon oxide (SiOx) or silicon nitride (SiNx) can be used for thesidewall 7021. As a method of forming the sidewall 7021 on the sidesurface of the gate electrode 7017, a method where a silicon oxide(SiOx) film or a silicon nitride (SiNx) film is formed after the gateelectrode 7017 is formed, and then, the silicon oxide (SiOx) film or thesilicon nitride (SiNx) film is etched by anisotropic etching can beused, for example. Thus, the silicon oxide (SiOx) film or the siliconnitride (SiNx) film remains only on the side surface of the gateelectrode 7017, so that the sidewall 7021 can be formed on the sidesurface of the gate electrode 7017.

Note that a conductive film may be provided under the insulating film7012. This conductive film functions as a common electrode in somecases.

FIG. 33 shows cross-sectional structures of a bottom-gate transistor anda capacitor.

A first insulating film (an insulating film 7092) is formed over anentire surface of a substrate 7091. Note that the structure is notlimited thereto, and the first insulating film (the insulating film7092) is not formed in some cases. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing properties of a transistor. That is, the firstinsulating film functions as a base film. Thus, a transistor with highreliability can be formed. As the first insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiOxNy), or the like can be used.

A first conductive layer (a conductive layer 7093 and a conductive layer7094) is formed over the first insulating film. The conductive layer7093 includes a portion functioning as a gate electrode of a transistor7108. The conductive layer 7094 includes a portion functioning as afirst electrode of a capacitor 7109. As the first conductive layer, Ti,Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, Ge, or thelike, or an alloy of these elements can be used. Further, a stackedlayer of these elements (including the alloy thereof) can be used.

A second insulating film (an insulating film 7104) is formed to cover atleast the first conductive layer. The second insulating film functionsas a gate insulating film. As the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiOxNy), or the like can be used.

Note that as a portion of the second insulating film, which is incontact with the semiconductor layer, a silicon oxide film is preferablyused. This is because the trap level at the interface between thesemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A semiconductor layer is formed in part of a portion over the secondinsulating film, which overlaps with the first conductive layer, by aphotolithography method, an ink-jet method, a printing method, or thelike. Part of the semiconductor layer extends to a portion over thesecond insulating film, which does not overlap with the first conductivelayer. The semiconductor layer includes a channel formation region (achannel formation region 7100), an LDD region (LDD regions 7098 and7099), and an impurity region (impurity regions 7095, 7096, and 7097).The channel formation region 7100 functions as a channel formationregion of the transistor 7108. The LDD regions 7098 and 7099 function asLDD regions of the transistor 7108. Note that the LDD regions 7098 and7099 are not always necessarily formed. The impurity region 7095includes a portion functioning as one of a source electrode and a drainelectrode of the transistor 7108. The impurity region 7096 includes aportion functioning as the other of the source electrode and the drainelectrode of the transistor 7108. The impurity region 7097 includes aportion functioning as a second electrode of the capacitor 7109.

A third insulating film (an insulating film 7101) is formed over theentire surface. A contact hole is selectively formed in part of thethird insulating film. The insulating film 7101 functions as aninterlayer film. As the third insulating film, an inorganic material(e.g., silicon oxide, silicon nitride, or silicon oxynitride), anorganic compound material having a low dielectric constant (e.g., aphotosensitive or nonphotosensitive organic resin material), or the likecan be used. Alternatively, a material containing siloxane may be used.Note that siloxane is a material in which a skeleton structure is formedby a bond of silicon (Si) and oxygen (O). As a substitute, an organicgroup containing at least hydrogen (such as an alkyl group or aromatichydrocarbon) is used. Alternatively, a fluoro group, or a fluoro groupand an organic group containing at least hydrogen may be used as asubstituent.

A second conductive layer (a conductive layer 7102 and a conductivelayer 7103) is formed over the third insulating film. The conductivelayer 7102 is connected to the other of the source electrode and thedrain electrode of the transistor 7108 through the contact hole formedin the third insulating film. Thus, the conductive layer 7102 includes aportion functioning as the other of the source electrode and the drainelectrode of the transistor 7108. When the conductive layer 7103 iselectrically connected to the conductive layer 7094, the conductivelayer 7103 includes a portion functioning as the first electrode of thecapacitor 7109. Alternatively, when the conductive layer 7103 iselectrically connected to the impurity region 7097 which is a conductivelayer, the conductive layer 7103 includes a portion functioning as thesecond electrode of the capacitor 7109. Further alternatively, when theconductive layer 7103 is not connected to the conductive layer 7094 andthe impurity region 7097, a capacitor other than the capacitor 7109 isformed. In this capacitor, the conductive layer 7103, the impurityregion 7097, and the insulating film 7101 are used as a first electrode,a second electrode, and an insulating film, respectively. Note that asthe second conductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt,Nb, Si, Zn, Fe, Ba, Ge, or the like, or an alloy of these elements canbe used. Further, a stacked layer of these elements (including the alloythereof) can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

Note that a conductive film may be provided under the insulating film7092. This conductive film functions as a common electrode in somecases.

Next, structures of a transistor and a capacitor are described when anamorphous silicon (a-Si:H) film, a microcrystalline silicon film, or thelike is used as a semiconductor layer of the transistor.

FIG. 30 shows cross-sectional structures of a top-gate transistor and acapacitor.

A first insulating film (an insulating film 7032) is formed over anentire surface of a substrate 7031. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

Note that the first insulating film is not always necessarily formed.When the first insulating film is not formed, reduction in the number ofsteps and manufacturing cost can be realized. Further, since thestructure can be simplified, the yield can be improved.

A first conductive layer (a conductive layer 7033, a conductive layer7034, and a conductive layer 7035) is formed over the first insulatingfilm. The conductive layer 7033 includes a portion functioning as one ofa source electrode and a drain electrode of a transistor 7048. Theconductive layer 7034 includes a portion functioning as the other of thesource electrode and the drain electrode of the transistor 7048. Theconductive layer 7035 includes a portion functioning as a firstelectrode of a capacitor 7049. As the first conductive layer, Ti, Mo,Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, Ge, or the like,or an alloy of these elements can be used. Further, a stacked layer ofthese elements (including the alloy thereof) can be used.

A first semiconductor layer (a semiconductor layer 7036 and asemiconductor layer 7037) is formed above the conductive layers 7033 and7034. The semiconductor layer 7036 includes a portion functioning as oneof the source electrode and the drain electrode. The semiconductor layer7037 includes a portion functioning as the other of the source electrodeand the drain electrode. As the first semiconductor layer, siliconcontaining phosphorus or the like can be used.

A second semiconductor layer (a semiconductor layer 7038) is formed overthe first insulating film and between the conductive layer 7033 and theconductive layer 7034. Part of the semiconductor layer 7038 extends overthe conductive layers 7033 and 7034. The semiconductor layer 7038includes a portion functioning as a channel region of the transistor7048. As the second semiconductor layer, a semiconductor layer having nocrystallinity such as amorphous silicon (a-Si:H), a semiconductor layersuch as a microcrystalline semiconductor (μ-Si:H), or the like can beused.

A second insulating film (an insulating film 7039 and an insulating film7040) is formed to cover at least the semiconductor layer 7038 and theconductive layer 7035. The second insulating film functions as a gateinsulating film. As the second insulating film, a single layer or astacked layer of a silicon oxide film, a silicon nitride film, a siliconoxynitride film (SiOxNy), or the like can be used.

Note that as a portion of the second insulating film, which is incontact with the second semiconductor layer, a silicon oxide film ispreferably used. This is because the trap level at the interface betweenthe second semiconductor layer and the second insulating film islowered.

Note that when the second insulating film is in contact with Mo, asilicon oxide film is preferably used as a portion of the secondinsulating film in contact with Mo. This is because the silicon oxidefilm does not oxidize Mo.

A second conductive layer (a conductive layer 7041 and a conductivelayer 7042) is formed over the second insulating film. The conductivelayer 7041 includes a portion functioning as a gate electrode of thetransistor 7048. The conductive layer 7042 functions as a secondelectrode of the capacitor 7049 or a wiring. As the second conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or the like, or an alloy of these elements can be used. Further, astacked layer of these elements (including the alloy thereof) can beused.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

Note that a conductive film may be provided under the insulating film7032. This conductive film functions as a common electrode in somecases.

FIG. 31 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorshown in FIG. 31 has a channel etch structure.

A first insulating film (an insulating film 7052) is formed over anentire surface of a substrate 7051. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

Note that the first insulating film is not always necessarily formed.When the first insulating film is not formed, reduction in the number ofsteps and manufacturing cost can be realized. Further, since thestructure can be simplified, the yield can be improved.

A first conductive layer (a conductive layer 7053 and a conductive layer7054) is formed over the first insulating film. The conductive layer7053 includes a portion functioning as a gate electrode of a transistor7068. The conductive layer 7054 includes a portion functioning as afirst electrode of a capacitor 7069. As the first conductive layer, Ti,Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, Ge, or thelike, or an alloy of these elements can be used. Further, a stackedlayer of these elements (including the alloy thereof) can be used.

A second insulating film (an insulating film 7055) is formed to cover atleast the first conductive layer. The second insulating film functionsas a gate insulating film. As the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiOxNy), or the like can be used.

Note that as a portion of the second insulating film, which is incontact with the semiconductor layer, a silicon oxide film is preferablyused. This is because the trap level at the interface between thesemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 7056) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an ink-jetmethod, a printing method, or the like. Part of the semiconductor layer7056 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer7056 includes a portion functioning as a channel region of thetransistor 7068. As the semiconductor layer 7056, a semiconductor layerhaving no crystallinity such as amorphous silicon (a-Si:H), asemiconductor layer such as a microcrystalline semiconductor (μ-Si:H),or the like can be used.

A second semiconductor layer (a semiconductor layer 7057 and asemiconductor layer 7058) is formed over part of the first semiconductorlayer. The semiconductor layer 7057 includes a portion functioning asone of a source electrode and a drain electrode. The semiconductor layer7058 includes a portion functioning as the other of the source electrodeand the drain electrode. As the second semiconductor layer, siliconcontaining phosphorus or the like can be used.

A second conductive layer (a conductive layer 7059, a conductive layer7060, and a conductive layer 7061) is formed over the secondsemiconductor layer and the second insulating film. The conductive layer7059 includes a portion functioning as one of a source electrode and adrain electrode of the transistor 7068. The conductive layer 7060includes a portion functioning as the other of the source electrode andthe drain electrode of the transistor 7068. The conductive layer 7061includes a portion functioning as a second electrode of the capacitor7069. As the second conductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag,Au, Pt, Nb, Si, Zn, Fe, Ba, Ge, or the like, or an alloy of theseelements can be used. Further, a stacked layer of these elements(including the alloy thereof) can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

Here, an example of a step of forming a channel etch type transistor isdescribed. The first semiconductor layer and the second semiconductorlayer can be formed using the same mask. Specifically, the firstsemiconductor layer and the second semiconductor layer are sequentiallyformed. At this time, the first semiconductor layer and the secondsemiconductor layer are formed using the same mask.

Another example of a step of forming a channel etch type transistor isdescribed. Without using an additional mask, a channel region of atransistor can be formed. Specifically, after the second conductivelayer is formed, part of the second semiconductor layer is removed usingthe second conductive layer as a mask. Alternatively, part of the secondsemiconductor layer is removed by using the same mask as the secondconductive layer. The first semiconductor layer below the removed secondsemiconductor layer functions as a channel region of the transistor.

Note that a conductive film may be provided under the insulating film7052. This conductive film functions as a common electrode in somecases.

FIG. 32 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorshown in FIG. 32 has a channel protection (channel stop) structure.

A first insulating film (an insulating film 7072) is formed over anentire surface of a substrate 7071. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

Note that the first insulating film is not always necessarily formed.When the first insulating film is not formed, reduction in the number ofsteps and manufacturing cost can be realized. Further, since thestructure can be simplified, the yield can be improved.

A first conductive layer (a conductive layer 7073 and a conductive layer7074) is formed over the first insulating film. The conductive layer7073 includes a portion functioning as a gate electrode of a transistor7088. The conductive layer 7074 includes a portion functioning as afirst electrode of a capacitor 7089. As the first conductive layer, Ti,Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, Ge, or thelike, or an alloy of these elements can be used. Further, a stackedlayer of these elements (including the alloy thereof) can be used.

A second insulating film (an insulating film 7075) is formed to cover atleast the first conductive layer. The second insulating film functionsas a gate insulating film. As the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiOxNy), or the like can be used.

Note that as a portion of the second insulating film, which is incontact with the semiconductor layer, a silicon oxide film is preferablyused. This is because the trap level at the interface between thesemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 7076) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an ink-jetmethod, a printing method, or the like. Part of the semiconductor layer7076 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer7076 includes a portion functioning as a channel region of thetransistor 7088. As the semiconductor layer 7076, a semiconductor layerhaving no crystallinity such as amorphous silicon (a-Si:H), asemiconductor layer such as a microcrystalline semiconductor (μ-Si:H),or the like can be used.

A third insulating film (an insulating film 7082) is formed over part ofthe first semiconductor layer. The insulating film 7082 has a functionto prevent the channel region of the transistor 7088 from being removedby etching. That is, the insulating film 7082 functions as a channelprotection film (a channel stop film). As the third insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

A second semiconductor layer (a semiconductor layer 7077 and asemiconductor layer 7078) is formed over part of the first semiconductorlayer and part of the third insulating film. The semiconductor layer7077 includes a portion functioning as one of a source electrode and adrain electrode. The semiconductor layer 7078 includes a portionfunctioning as the other of the source electrode and the drainelectrode. As the second semiconductor layer, silicon containingphosphorus or the like can be used.

A second conductive layer (a conductive layer 7079, a conductive layer7080, and a conductive layer 7081) is formed over the secondsemiconductor layer. The conductive layer 7079 includes a portionfunctioning as one of the source electrode and the drain electrode ofthe transistor 7088. The conductive layer 7080 includes a portionfunctioning as the other of the source electrode and the drain electrodeof the transistor 7088. The conductive layer 7081 includes a portionfunctioning as a second electrode of the capacitor 7089. As the secondconductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, Ge, or the like, or an alloy of these elements can be used.Further, a stacked layer of these elements (including the alloy thereof)can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

Note that a conductive film may be provided under the insulating film7072. This conductive film functions as a common electrode in somecases.

Next, an example where a semiconductor substrate is used as a substratefor a transistor. Since a transistor formed using a semiconductorsubstrate has high mobility, the size of the transistor can bedecreased. Accordingly, the number of transistors per unit area can beincreased (the degree of integration can be improved), and the size ofthe substrate can be decreased as the degree of integration is increasedin the case of the same circuit structure. Thus, manufacturing cost canbe reduced. Further, since the circuit scale can be increased as thedegree of integration is increased in the case of the same substratesize, more advanced function can be provided without increase inmanufacturing cost. Moreover, reduction in variations in characteristicscan improve manufacturing yield. Reduction in operating voltage canreduce power consumption. High mobility can realize high-speedoperation.

When a circuit which is formed by integrating transistors formed using asemiconductor substrate is mounted on a device in the form of an IC chipor the like, the circuit can provide the device with various functions.For example, a peripheral driver circuit (e.g., a data driver (a sourcedriver), a scan driver (a gate driver), a timing controller, an imageprocessing circuit, an interface circuit, a power supply circuit, or anoscillation circuit) of a display device is formed by integratingtransistors formed using a semiconductor substrate, so that a smallperipheral circuit which can operate with low power consumption and athigh speed can be formed at low cost in high yield. Note that a circuitwhich is formed by integrating transistors formed using a semiconductorsubstrate may have a unipolar transistor. Thus, a manufacturing processcan be simplified, so that manufacturing cost can be reduced.

A circuit which is formed by integrating transistors formed using asemiconductor substrate may also be used for a display panel, forexample. More specifically, the circuit can be used for a reflectiveliquid crystal panel such as a liquid crystal on silicon (LCOS) device,a digital micromirror device (DMD) in which micromirrors are integrated,an EL panel, and the like. When such a display panel is formed using asemiconductor substrate, a small display panel which can operate withlow power consumption and at high speed can be formed at low cost inhigh yield. Note that the display panel may be formed over an elementhaving a function other than a function to drive the display panel, suchas a large-scale integration (LSI).

Note that a structure of a transistor is not limited to the structureshown in each drawing. For example, a transistor may have an inverselystaggered structure, a FinFET structure, or the like. It is preferableto have a FinFET structure since a short channel effect due tominiaturization of transistor size can be suppressed.

The above is the description of the structures and manufacturing methodsof transistors. Here, a wiring, an electrode, a conductive layer, aconductive film, a terminal, a via, a plug, and the like are preferablyformed of one or more elements selected from aluminum (Al), tantalum(Ta), titanium (Ti), molybdenum (Mo), tungsten (W), neodymium (Nd),chromium (Cr), nickel (Ni), platinum (Pt), gold (Au), silver (Ag),copper (Cu), magnesium (Mg), scandium (Sc), cobalt (Co), zinc (Zn),niobium (Nb), silicon (Si), phosphorus (P), boron (B), arsenic (As),gallium (Ga), indium (In), tin (Sn), and oxygen (O); or a compound or analloy material including one or more of the aforementioned elements(e.g., indium tin oxide (ITO), indium zinc oxide (IZO), indium tin oxidecontaining silicon (ITSO), zinc oxide (ZnO), tin oxide (SnO), cadmiumtin oxide (CTO), aluminum neodymium (Al—Nd), magnesium silver (Mg—Ag),or molybdenum-niobium (Mo—Nb)); a substance in which these compounds arecombined; or the like. Alternatively, they are preferably formed tocontain a substance including a compound (silicide) of silicon and oneor more of the aforementioned elements (e.g., aluminum silicon,molybdenum silicon, or nickel silicide); or a compound of nitrogen andone or more of the aforementioned elements (e.g., titanium nitride,tantalum nitride, or molybdenum nitride).

Note that silicon (Si) may contain an n-type impurity (such asphosphorus) or a p-type impurity (such as boron). When silicon containsthe impurity, the conductivity is increased, and a function similar to ageneral conductor can be realized. Thus, such silicon can be utilizedeasily as a wiring, an electrode, or the like.

In addition, silicon with various levels of crystallinity, such assingle crystalline silicon, polycrystalline silicon, or microcrystallinesilicon can be used. Alternatively, silicon having no crystallinity,such as amorphous silicon can be used. By using single crystallinesilicon or polycrystalline silicon, resistance of a wiring, anelectrode, a conductive layer, a conductive film, a terminal, or thelike can be reduced. By using amorphous silicon or microcrystallinesilicon, a wiring or the like can be formed by a simple process.

Aluminum and silver have high conductivity, and thus can reduce a signaldelay. Further, since aluminum and silver can be easily etched, they canbe minutely processed.

Copper has high conductivity, and thus can reduce a signal delay. Whencopper is used, a stacked-layer structure is preferably employed toimprove adhesion.

Molybdenum and titanium are preferable since even if molybdenum ortitanium is in contact with an oxide semiconductor (e.g., ITO or IZO) orsilicon, molybdenum or titanium does not cause defects. Further,molybdenum and titanium are preferable since they are easily etched andhas high heat resistance.

Tungsten is preferable since it has an advantage such as high heatresistance.

Neodymium is also preferable since it has an advantage such as high heatresistance. In particular, an alloy of neodymium and aluminum ispreferable since heat resistance is increased and aluminum hardly causeshillocks.

Silicon is preferable since it can be formed at the same time as asemiconductor layer included in a transistor and has high heatresistance.

Since ITO, IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO),and cadmium tin oxide (CTO) have light-transmitting properties, they canbe used as a portion which transmits light. For example, they can beused for a pixel electrode or a common electrode.

IZO is preferable since it is easily etched and processed. In etchingIZO, a residue is hardly left. Thus, when IZO is used for a pixelelectrode, defects (such as short circuit or orientation disorder) of aliquid crystal element or a light-emitting element can be reduced.

A wiring, an electrode, a conductive layer, a conductive film, aterminal, a via, a plug, or the like may have a single-layer structureor a multi-layer structure. By employing a single-layer structure, eachmanufacturing process of a wiring, an electrode, a conductive layer, aconductive film, a terminal, or the like can be simplified, the numberof days for a process can be reduced, and cost can be reduced.Alternatively, by employing a multi-layer structure, a wiring, anelectrode, and the like with high quality can be formed while anadvantage of each material is utilized and a disadvantage thereof isreduced. For example, when a low-resistant material (e.g., aluminum) isincluded in a multi-layer structure, reduction in resistance of a wiringcan be realized. As another example, when a stacked-layer structurewhere a low heat-resistant material is interposed between highheat-resistant materials is employed, heat resistance of a wiring, anelectrode, and the like can be increased, utilizing advantages of thelow heat-resistance material. For example, it is preferable to employ astacked-layer structure where a layer containing aluminum is interposedbetween layers containing molybdenum, titanium, neodymium, or the like.

When wirings, electrodes, or the like are in direct contact with eachother, they adversely affect each other in some cases. For example, onewiring or one electrode is mixed into a material of another wiring oranother electrode and changes its properties, and thus, an intendedfunction cannot be obtained in some cases. As another example, when ahigh-resistant portion is formed, a problem may occur so that it cannotbe normally formed. In such cases, a reactive material is preferablyinterposed by or covered with a non-reactive material in a stacked-layerstructure. For example, when ITO and aluminum are connected, titanium,molybdenum, or an alloy of neodymium is preferably interposed betweenITO and aluminum. As another example, when silicon and aluminum areconnected, titanium, molybdenum, or an alloy of neodymium is preferablyinterposed between silicon and aluminum.

The term “wiring” indicates a portion including a conductor. A wiringmay be a linear shape or may be short without a linear shape. Therefore,an electrode is included in a wiring.

Note that a carbon nanotube may be used for a wiring, an electrode, aconductive layer, a conductive film, a terminal, a via, a plug, or thelike. Since a carbon nanotube has a light-transmitting property, it canbe used for a portion which transmits light. For example, a carbonnanotube can be used for a pixel electrode or a common electrode.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 8

In this embodiment mode, a structure of a display device is described.

A structure of a display device is described with reference to FIG. 34A.FIG. 34A is a top plan view of the display device.

A pixel portion 8201, a scan line input terminal 8203, and a signal lineinput terminal 8204 are formed over a substrate 8200. Scan linesextending in a row direction from the scan line input terminal 8203 areformed over the substrate 8200, and signal lines extending in a columndirection from the signal line input terminal 8204 are formed over thesubstrate 8200. Pixels 8202 are arranged in matrix at each intersectionof the scan lines and the signal lines in the pixel portion 8201.

The above is the description of the case where a signal is input from anexternal driver circuit; however, the invention is not limited thereto,and an IC chip can be mounted on a display device.

For example, as shown in FIG. 35A, an IC chip 8211 can be mounted on thesubstrate 8200 by a COG (Chip On Glass) method. In this case, the ICchip 8211 can be examined before being mounted on the substrate 8200, sothat improvement in yield and reliability of the display device can berealized. Note that portions common to those in FIG. 34A are denoted bycommon reference numerals, and description thereof is omitted.

As another example, as shown in FIG. 35B, the IC chip 8211 can bemounted on an FPC (Flexible Printed Circuit) 8210 by a TAB (TapeAutomated Bonding) method. In this case, the IC chip 8211 can beexamined before being mounted on the FPC 8210, so that improvement inyield and reliability of the display device can be realized. Note thatportions common to those in FIG. 34A are denoted by common referencenumerals, and description thereof is omitted.

Not only the IC chip can be mounted on the substrate 8200, but also adriver circuit can be formed over the substrate 8200.

For example, as shown in FIG. 34B, a scan line driver circuit 8205 canbe formed over the substrate 8200. In this case, the cost can be reducedby reduction in the number of components. Further, reliability can beimproved by reduction in the number of connection points betweencomponents. Since the driving frequency of the scan line driver circuit8205 is low, the scan line driver circuit 8205 can be easily formedusing amorphous silicon or microcrystalline silicon as a semiconductorlayer of a transistor. Note that an IC chip for outputting a signal tothe signal line may be mounted on the substrate 8200 by a COG method.Alternatively, an FPC on which an IC chip for outputting a signal to thesignal line is mounted by a TAB method may be provided on the substrate8200. In addition, an IC chip for controlling the scan line drivercircuit 8205 may be mounted on the substrate 8200 by a COG method.Alternatively, an FPC on which an IC chip for controlling the scan linedriver circuit 8205 is mounted by a TAB method may be provided on thesubstrate 8200. Note that portions common to those in FIG. 34A aredenoted by common reference numerals, and description thereof isomitted.

As another example, as shown in FIG. 34C, the scan line driver circuit8205 and a signal line driver circuit 8206 can be formed over thesubstrate 8200. Thus, the cost can be reduced by reduction in the numberof components. Further, reliability can be improved by reduction in thenumber of connection points between components. Note that an IC chip forcontrolling the scan line driver circuit 8205 may be mounted on thesubstrate 8200 by a COG method. Alternatively, an FPC on which an ICchip for controlling the scan line driver circuit 8205 is mounted by aTAB method may be provided on the substrate 8200. In addition, an ICchip for controlling the signal line driver circuit 8206 may be mountedon the substrate 8200 by a COG method. Alternatively, an FPC on which anIC chip for controlling the signal line driver circuit 8206 is mountedby a TAB method may be provided on the substrate 8200. Note thatportions common to those in FIG. 34A are denoted by common referencenumerals, and description thereof is omitted.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 9

In this embodiment mode, an operation of a display device is described.

FIG. 36 shows a structure example of a display device.

A display device 8400 includes a pixel portion 8401, a signal linedriver circuit 8403, and a scan line driver circuit 8404. In the pixelportion 8401, a plurality of signal lines S1 to Sn extend from thesignal line driver circuit 8403 in a column direction. In the pixelportion 8401, a plurality of scan lines G1 to Gm extend from the scanline driver circuit 8404 in a row direction. Pixels 8402 are arranged inmatrix at each intersection of the plurality of signal lines S1 to Snand the plurality of scan lines G1 to Gm.

The signal line driver circuit 8403 has a function to output a signal toeach of the signal lines S1 to Sn. This signal may be referred to as avideo signal. The scan line driver circuit 8404 has a function to outputa signal to each of the scan lines G1 to Gm. This signal may be referredto as a scan signal.

The pixel 8402 may include at least a switching element connected to thesignal line. On/off of the switching element is controlled by apotential of a scan line (a scan signal). When the switching element isturned on, the pixel 8402 is selected. On the other hand, when theswitching element is turned off, the pixel 8402 is not selected.

When the pixel 8402 is selected (a selection state), a video signal isinput to the pixel 8402 from the signal line. A state (e.g., luminance,transmittance, or voltage of a storage capacitor) of the pixel 8402 ischanged in accordance with the video signal input.

When the pixel 8402 is not selected (a non-selection state), the videosignal is not input to the pixel 8402. Note that the pixel 8402 holds apotential corresponding to the video signal which is input whenselected; thus, the pixel 8402 maintains the state (e.g., luminance,transmittance, or voltage of a storage capacitor) in accordance with thevideo signal.

A structure of the display device is not limited to that shown in FIG.36. For example, an additional wiring (such as a scan line, a signalline, a power supply line, a capacitor line, or a common line) may beadded in accordance with the structure of the pixel 8402. As anotherexample, a circuit having various functions may be added.

FIG. 37 shows an example of a timing chart for describing an operationof a display device.

The timing chart of FIG. 37 shows one frame period corresponding to aperiod when an image of one screen is displayed. On one frame period isnot particularly limited, but one frame period is preferably 1/60 secondor less so that a viewer does not perceive a flicker.

The timing chart of FIG. 37 shows timing for selecting the scan line G1in the first row, the scan line Gi (one of the scan lines G1 to Gm) inthe i-th row, the scan line Gi+1 in the (i+1)th row, and the scan lineGm in the m-th row.

At the same time as the scan line is selected, the pixel 8402 connectedto the scan line is also selected. For example, when the scan line Gi inthe i-th row is selected, the pixel 8402 connected to the scan line Giin the i-th row is also selected.

The scan lines G1 to Gm are sequentially selected (also referred to asscanned) from the scan line G1 in the first row to the scan line Gm inthe m-th row. For example, while the scan line Gi in the i-th row isselected, the scan lines (G1 to Gi−1 and Gi+1 to Gm) other than the scanline Gi in the i-th row are not selected. Then, during the next period,the scan line Gi+1 in the (i+1)th row is selected. The period duringwhich one scan line is selected is referred to as one gate selectionperiod.

Therefore, when a scan line in a certain row is selected, a plurality ofpixels 8402 connected to the scan line receive a video signal from eachof the signal lines S1 to Sn. For example, when the scan line Gi in thei-th row is selected, the plurality of pixels 8402 connected to the scanline Gi in the i-th row receive a given video signal from each of thesignal lines S1 to Sn. Thus, each of the plurality of pixels 8402 can becontrolled individually by the scan signal and the video signal.

Next, the case where one gate selection period is divided into aplurality of subgate selection periods is described.

FIG. 38 is a timing chart in the case where one gate selection period isdivided into two subgate selection periods (a first subgate selectionperiod and a second subgate selection period).

Note that one gate selection period may be divided into three or moresubgate selection periods.

The timing chart of FIG. 38 shows one frame period corresponding to aperiod when an image of one screen is displayed. One frame period is notparticularly limited, but one frame period is preferably 1/60 second orless so that a viewer does not perceive a flicker.

Note that one frame is divided into two subframes (a first subframe anda second subframe).

The timing chart of FIG. 38 shows timing for selecting the scan line Giin the i-th row, the scan line Gi+1 in the (i+1)th row, the scan line Gj(one of the scan lines Gi+1 to Gm) in the j-th row, and the scan lineGj+1 in the (j+1)th row.

At the same time as the scan line is selected, the pixel 8402 connectedto the scan line is also selected. For example, when the scan line Gi inthe i-th row, the pixel 8402 connected to the scan line Gi in the i-throw is also selected.

The scan lines G1 to Gm are sequentially scanned in each subgateselection period. For example, in a certain one gate selection period,the scan line Gi in the i-th row is selected in the first subgateselection period, and the scan line Gj in the j-th row is selected inthe second subgate selection period. Thus, in one gate selection period,an operation can be performed as if the scan signals of two rows areselected. At this time, different video signals are input to the signallines S1 to Sn in the first subgate selection period and the secondsubgate selection period. Accordingly, a plurality of pixels 8402connected to the scan line Gi in the i-th row can receive a differentvideo signal from a plurality of pixels 8402 connected to the scan lineGj in the j-th row.

Next, a driving method for displaying images with high quality isdescribed.

FIGS. 39A and 39B are views for describing high frequency driving.

FIG. 39A shows the case where an interpolation image is displayedbetween two input images. A period 8410 is a cycle of an input imagesignal. An image 8411, an image 8412, an image 8413, and an image 8414are a first input image, a first interpolation image, a second inputimage, and a second interpolation image, respectively. Here, the inputimage corresponds to an image based on a signal input from outside of adisplay device. Further, the interpolation image corresponds to an imagewhich is displayed at a timing different from that of the input image soas to interpolate an image.

The image 8412 is an image formed based on image signals of the images8411 and 8413. Specifically, movement of an object is estimated bydifference between a position of the object included in the image 8411and a position of the object included in the image 8413, and the imagecan be made in which the position of the object included in the image8412 is at an intermediate state between the image 8411 and the image8413. This process is referred to as motion compensation. Since theimage 8412 is formed by motion compensation, the object at theintermediate (½) position, which cannot be displayed only by the inputimage, can be displayed, and the movement of the object can be smoothedto be displayed. Alternatively, the image 8412 can be formed by anaverage value of the image signals of the images 8411 and 8413. Thus,the load to a circuit due to formation of the interpolation image can bereduced, so that power consumption can be reduced.

Alternatively, the image 8412 can be formed from the image 8411.Specifically, the image 8412 can be formed by increasing or decreasingbrightness of the image 8411 entirely or partially. More specifically,the image with the entire brightness higher or lower can be made byconverting gamma characteristics of the image 8411.

Note that the image 8412 may be a black image. Thus, the quality of amoving image in a hold-type display device can be improved.

FIG. 39B shows the case where two interpolation images are displayedbetween two input images. The period 8410 is a cycle of an input imagesignal. An image 8421, an image 8422, an image 8423, and an image 8424are a first input image, a first interpolation image, a secondinterpolation image, and a second input image, respectively.

Each of the image 8422 and the image 8423 can be formed based on imagesignals of the images 8421 and 8424. Specifically, the images 8422 and8423 can be formed by motion compensation using difference between aposition of an object included in the image 8421 and a position of theobject included in the image 8424. Since the images 8422 and 8423 areformed by motion compensation, the object at the intermediate (⅓ and ⅔)positions, which cannot be displayed only by the input image, can bedisplayed, movement of the object can be smoothed to be displayed. Inaddition, the images 8422 and 8423 can be formed by an average value ofthe image signals of the images 8421 and 8424. Thus, the load to acircuit due to formation of the interpolation image can be reduced, sothat power consumption can be reduced.

Alternatively, the images 8422 and 8423 can be formed from the images8421 and 8424. Specifically, the images 8422 and 8423 can be formed byincreasing or decreasing brightness of the image 8421 entirely orpartially. More specifically, the image with the entire brightnesshigher or lower can be made by converting the gamma characteristics ofthe image 8411.

Note that the images 8422 and 8423 may be black images. Thus, thequality of a moving image in a hold-type display device can be improved.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 10

In this embodiment mode, a peripheral portion of a liquid crystal panelis described.

FIG. 40 shows an example of a liquid crystal display device including aso-called edge-light type backlight unit 5201 and a liquid crystal panel5207. An edge-light type corresponds to a type in which a light sourceis provided at an end of a backlight unit and fluorescence of the lightsource is emitted from the entire light-emitting surface. The edge-lighttype backlight unit is thin and can save power.

The backlight unit 5201 includes a diffusion plate 5202, a light guideplate 5203, a reflection plate 5204, a lamp reflector 5205, and a lightsource 5206.

The light source 5206 has a function to emit light as necessary. As thelight source 5206, a cold cathode fluorescent lamp, a hot cathodefluorescent lamp, a light-emitting diode, an inorganic EL element, anorganic EL element, or the like can be used, for example.

FIGS. 41A to 41D each show a detailed structure of the edge-light typebacklight unit. Note that description of a diffusion plate, a lightguide plate, a reflection plate, and the like is omitted.

A backlight unit 5111 shown in FIG. 41A has a structure in which a coldcathode fluorescent lamp 5113 is used as a light source. A lampreflector 5112 is provided to efficiently reflect light from the coldcathode fluorescent lamp 5113. Such a structure is often used for alarge display device because luminance from the cold cathode fluorescentlamp is high.

A backlight unit 5221 shown in FIG. 41B has a structure in whichlight-emitting diodes (LEDs) 5223 are used as light sources. Forexample, the light-emitting diodes (LEDs) 5223 which emit white lightare provided at a predetermined interval. Further, a lamp reflector 5222is provided to efficiently reflect light from the light-emitting diodes(LEDs) 5223.

A backlight unit 5231 shown in FIG. 41C has a structure in whichlight-emitting diodes (LEDs) 5233, light-emitting diodes (LEDs) 5234,and light-emitting diodes (LEDs) 5235 of each color of RGB are used aslight sources. The light-emitting diodes (LEDs) 5233, 5234, and 5235 ofeach color of RGB are provided at a predetermined interval. By using thelight-emitting diodes (LEDs) 5233, 5234, and 5235 of each color of RGB,color reproducibility can be improved. In addition, a lamp reflector5232 is provided to efficiently reflect light from the light-emittingdiodes.

A backlight unit 5241 shown in FIG. 41D has a structure in whichlight-emitting diodes (LEDs) 5243, light-emitting diodes (LEDs) 5244,and light-emitting diodes (LEDs) 5245 of each color of RGB are used aslight sources. For example, among the light-emitting diodes (LEDs) 5243,5244, and 5245 of each color of RGB, the light-emitting diodes of acolor with low emission intensity (e.g., green) are provided more thanother light-emitting diodes. By using the light-emitting diodes (LEDs)5243, 5244, and 5245 of each color of RGB, color reproducibility can beimproved. In addition, a lamp reflector 5242 is provided to efficientlyreflect light from the light-emitting diodes.

FIG. 44 shows an example of a liquid crystal display device including aso-called direct-type backlight unit and a liquid crystal panel. Adirect type corresponds to a type in which a light source is provideddirectly under a light-emitting surface and fluorescence of the lightsource is emitted from the entire light-emitting surface. Thedirect-type backlight unit can efficiently utilize the amount of emittedlight.

A backlight unit 5290 includes a diffusion plate 5291, a light-shieldingplate 5292, a lamp reflector 5293, and a light source 5294.

The light source 5294 has a function to emit light as necessary. As thelight source 5294, a cold cathode fluorescent lamp, a hot cathodefluorescent lamp, a light-emitting diode, an inorganic EL element, anorganic EL element, or the like can be used, for example.

FIG. 42 shows an example of a structure of a polarizing plate (alsoreferred to as a polarizing film).

A polarizing film 5250 includes a protective film 5251, a substrate film5252, a PVA polarizing film 5253, a substrate film 5254, an adhesivelayer 5255, and a mold release film 5256.

When the PVA polarizing film 5253 is interposed between films (thesubstrate films 5252 and 5254) to be base materials, reliability can beimproved. Note that the PVA polarizing film 5253 may be interposed bytriacetyl cellulose (TAC) films with high light-transmitting propertiesand high durability. The substrate films and the TAC films each functionas a protective film of a polarizer included in the PVA polarizing film5253.

The adhesive layer 5255 which is to be attached to a glass substrate ofthe liquid crystal panel is attached to one of the substrate films (thesubstrate film 5254). Note that the adhesive layer 5255 is formed byapplying an adhesive to one of the substrate films (the substrate film5254). The adhesive layer 5255 is provided with the mold release film5256 (a separate film).

The other of the substrates films (the substrate film 5252) is providedwith the protective film 5251.

A hard coating scattering layer (an anti-glare layer) may be provided ona surface of the polarizing film 5250. Since the surface of the hardcoating scattering layer has minute unevenness formed by AG treatmentand has an anti-glare function which scatters external light, reflectionof external light in the liquid crystal panel and surface reflection canbe prevented.

A treatment in which a plurality of optical thin film layers havingdifferent refractive indexes are layered (also referred to asanti-reflection treatment or AR treatment) may be performed on thesurface of the polarizing film 5250. The plurality of layered opticalthin film layers having different refractive indexes can reducereflectivity on the surface by an interference effect of light.

FIGS. 43A to 43C show examples of a system block of a liquid crystaldisplay device.

In a pixel portion 5265, signal lines 5269 which are extended from asignal line driver circuit 5263 are provided. In the pixel portion 5265,scan lines 5260 which are extended from a scan line driver circuit 5264are also provided. Further, a plurality of pixels are arranged in matrixat intersections of the signal lines 5269 and the scan lines 5260. Notethat each of the plurality of pixels includes a switching element.Therefore, voltage for controlling inclination of liquid crystalmolecules can be separately input to each of the plurality of pixels. Astructure in which a switching element is provided at each intersectionin this manner is referred to as an active matrix type. Note that theinvention is not limited to such an active matrix type, and a structureof a passive matrix type may be used. In a passive matrix type, aswitching element is not included in each pixel, so that a process issimple.

A driver circuit portion 5268 includes a control circuit 5262, thesignal line driver circuit 5263, and the scan line driver circuit 5264.An image signal 5261 is input to the control circuit 5262. The controlcircuit 5262 controls the signal line driver circuit 5263 and the scanline driver circuit 5264 in accordance with the image signal 5261.Accordingly, the control circuit 5262 inputs a control signal to each ofthe signal line driver circuit 5263 and the scan line driver circuit5264. Then, in accordance with the control signal, the signal linedriver circuit 5263 inputs a video signal to each of the signal lines5269 and the scan line driver circuit 5264 inputs a scan signal to eachof the scan lines 5260. Then, the switching element included in thepixel is selected in accordance with the scan signal, and the videosignal is input to a pixel electrode of the pixel.

In addition, the control circuit 5262 also controls a power supply 5267in accordance with the image signal 5261. The power supply 5267 includesa means to supply power to a lighting unit 5266. As the lighting unit5266, an edge-light type backlight unit or a direct-type backlight unitcan be used. Note that a front light may be used as the lighting unit5266. A front light corresponds to a plate-like lighting unit includinga luminous body and a light conducting body, which is attached to thefront surface side of a pixel portion and illuminates the whole area.Such a lighting unit can uniformly illuminate the pixel portion at lowpower consumption.

As shown in FIG. 43B, the scan line driver circuit 5264 includes a shiftregister 5271, a level shifter 5272, and a circuit functioning as abuffer 5273. A signal such as a gate start pulse (GSP) or a gate clocksignal (GCK) is input to the shift register 5271.

As shown in FIG. 43C, the signal line driver circuit 5263 includes ashift register 5281, a first latch 5282, a second latch 5283, a levelshifter 5284, and a circuit functioning as a buffer 5285. The circuitfunctioning as the buffer 5285 corresponds to a circuit which has afunction to amplify a weak signal and includes an operational amplifieror the like. A signal such as a start pulse (SSP) is input to the shiftregister 5281, and data (DATA) such as a video signal is input to thefirst latch 5282. A latch (LAT) signal can be temporally held in thesecond latch 5283 and is simultaneously input to the pixel portion 5265.This is referred to as line sequential driving. Therefore, when a pixelin which not line sequential driving but dot sequential driving isperformed is employed, the second latch can be omitted.

In this embodiment mode, various types of liquid crystal panels can beused. For example, a structure in which a liquid crystal layer is sealedbetween two substrates can be used for the liquid crystal panel. Atransistor, a capacitor, a pixel electrode, an alignment film, or thelike is formed over one substrate. A polarizing plate, a retardationplate, or a prism sheet may be provided on the surface opposite to a topsurface of one substrate. A color filter, a black matrix, an oppositeelectrode, an alignment film, or the like is provided on the othersubstrate. A polarizing plate or a retardation plate may be provided onthe surface opposite to a top surface of the other substrate. Note thatthe color filter and the black matrix may be formed over the top surfaceof one substrate. In addition, three-dimensional display can beperformed by providing a slit (a grid) on the top surface or the surfaceopposite to the top surface of one substrate.

Each of the polarizing plate, the retardation plate, and the prism sheetcan be provided between the two substrates. Alternatively, each of thepolarizing plate, the retardation plate, and the prism sheet can beintegrated with one of the two substrates.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 11

In this embodiment mode, a structure and an operation of a pixel whichcan be applied to a liquid crystal display device are described.

In this embodiment mode, as an operation mode of a liquid crystalelement, a TN (Twisted Nematic) mode, an IPS (In-Plane-Switching) mode,an FFS (Fringe Field Switching) mode, an MVA (Multi-domain VerticalAlignment) mode, a PVA (Patterned Vertical Alignment) mode, an ASM(Axially Symmetric aligned Microcell) mode, an OCB (Optical CompensatedBirefringence) mode, an FLC (Ferroelectric Liquid Crystal) mode, an AFLC(AntiFerroelectric Liquid Crystal) mode, or the like can be used.

FIG. 45A shows an example of a pixel structure which can be applied tothe liquid crystal display device.

A pixel 5600 includes a transistor 5601, a liquid crystal element 5602,and a capacitor 5603. A gate of the transistor 5601 is connected to awiring 5605. A first terminal of the transistor 5601 is connected to awiring 5604. A second terminal of the transistor 5601 is connected to afirst electrode of the liquid crystal element 5602 and a first electrodeof the capacitor 5603. A second electrode of the liquid crystal element5602 corresponds to an opposite electrode 5607. A second electrode ofthe capacitor 5603 is connected to a wiring 5606.

The wiring 5604 functions as a signal line. The wiring 5605 functions asa scan line. The wiring 5606 functions as a capacitor line. Thetransistor 5601 functions as a switch. The capacitor 5603 functions as astorage capacitor.

It is only necessary that the transistor 5601 function as a switch, andthe transistor 5601 may be a p-channel transistor or an n-channeltransistor.

FIG. 45B shows an example of a pixel structure which can be applied tothe liquid crystal display device. In particular, FIG. 45B shows anexample of a pixel structure which can be applied to a liquid crystaldisplay device suitable for a lateral electric field mode (including anIPS mode and an FFS mode).

A pixel 5610 includes a transistor 5611, a liquid crystal element 5612,and a capacitor 5613. A gate of the transistor 5611 is connected to awiring 5615. A first terminal of the transistor 5611 is connected to awiring 5614. A second terminal of the transistor 5611 is connected to afirst electrode of the liquid crystal element 5612 and a first electrodeof the capacitor 5613. A second electrode of the liquid crystal element5612 is connected to a wiring 5616. A second electrode of the capacitor5613 is connected to the wiring 5616.

The wiring 5614 functions as a signal line. The wiring 5615 functions asa scan line. The wiring 5616 functions as a capacitor line. Thetransistor 5611 functions as a switch. The capacitor 5613 functions as astorage capacitor.

It is only necessary that the transistor 5611 function as a switch, andthe transistor 5611 may be a p-channel transistor or an n-channeltransistor.

FIG. 46 shows an example of a pixel structure which can be applied tothe liquid crystal display device. In particular, FIG. 46 shows anexample of a pixel structure in which an aperture ratio of a pixel canbe increased by reducing the number of wirings.

FIG. 46 shows two pixels (a pixel 5620 and a pixel 5630) which areprovided in the same column direction. For example, when the pixel 5620is provided in the N-th row, the pixel 5630 is provided in the (N+1)throw.

A pixel 5620 includes a transistor 5621, a liquid crystal element 5622,and a capacitor 5623. A gate of the transistor 5621 is connected to awiring 5625. A first terminal of the transistor 5621 is connected to awiring 5624. A second terminal of the transistor 5621 is connected to afirst electrode of the liquid crystal element 5622 and a first electrodeof the capacitor 5623. A second electrode of the liquid crystal element5622 corresponds to an opposite electrode 5627. A second electrode ofthe capacitor 5623 is connected to a wiring which is the same as thatconnected to a gate of a transistor in the previous row.

A pixel 5630 includes a transistor 5631, a liquid crystal element 5632,and a capacitor 5633. A gate of the transistor 5631 is connected to awiring 5635. A first terminal of the transistor 5631 is connected to thewiring 5624. A second terminal of the transistor 5631 is connected to afirst electrode of the liquid crystal element 5632 and a first electrodeof the capacitor 5633. A second electrode of the liquid crystal element5632 corresponds to an opposite electrode 5637. A second electrode ofthe capacitor 5633 is connected to a wiring which is the same as thatconnected to the gate of the transistor in the previous row (i.e., thewiring 5625).

The wiring 5624 functions as a signal line. The wiring 5625 functions asa scan line of the N-th row, and also as a capacitor line of the (N+1)throw. The transistor 5621 functions as a switch. The capacitor 5623functions as a storage capacitor.

The wiring 5635 functions as a scan line of the (N+1)th row, and also asa capacitor line of the (N+2)th row. The transistor 5631 functions as aswitch. The capacitor 5633 functions as a storage capacitor.

It is only necessary that each of the transistor 5621 and the transistor5631 function as a switch, and each of the transistor 5621 and thetransistor 5631 may be a p-channel transistor or an n-channeltransistor.

FIG. 47 shows an example of a pixel structure which can be applied tothe liquid crystal display device. In particular, FIG. 47 shows anexample of a pixel structure in which a viewing angle can be improved byusing a subpixel.

A pixel 5659 includes a subpixel 5640 and a subpixel 5650. Although thecase where the pixel 5659 includes two subpixels is described below, thepixel 5659 may include three or more subpixels.

The subpixel 5640 includes a transistor 5641, a liquid crystal element5642, and a capacitor 5643. A gate of the transistor 5641 is connectedto a wiring 5645. A first terminal of the transistor 5641 is connectedto a wiring 5644. A second terminal of the transistor 5641 is connectedto a first electrode of the liquid crystal element 5642 and a firstelectrode of the capacitor 5643. A second electrode of the liquidcrystal element 5642 corresponds to an opposite electrode 5647. A secondelectrode of the capacitor 5643 is connected to a wiring 5646.

The subpixel 5650 includes a transistor 5651, a liquid crystal element5652, and a capacitor 5653. A gate of the transistor 5651 is connectedto a wiring 5655. A first terminal of the transistor 5651 is connectedto the wiring 5644. A second terminal of the transistor 5651 isconnected to a first electrode of the liquid crystal element 5652 and afirst electrode of the capacitor 5653. A second electrode of the liquidcrystal element 5652 corresponds to an opposite electrode 5657. A secondelectrode of the capacitor 5653 is connected to the wiring 5646.

The wiring 5644 functions as a signal line. The wiring 5645 functions asa scan line. The wiring 5655 functions as a scan line. The wiring 5646functions as a capacitor line. The transistor 5641 functions as aswitch. The transistor 5651 functions as a switch. The capacitor 5643functions as a storage capacitor. The capacitor 5653 functions as astorage capacitor.

It is only necessary that the transistor 5641 function as a switch, andthe transistor 5641 may be a p-channel transistor or an n-channeltransistor. It is only necessary that the transistor 5651 function as aswitch, and the transistor 5651 may be a p-channel transistor or ann-channel transistor.

A video signal input to the subpixel 5640 may be a value different fromthat of a video signal input to the subpixel 5650. In this case, theviewing angle can be widened because alignment of liquid crystalmolecules of the liquid crystal element 5642 can be different fromalignment of liquid crystal molecules of the liquid crystal element5652.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detailed, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 12

In this embodiment mode, a method for driving a display device isdescribed. In particular, a method for driving a liquid crystal displaydevice is described.

A liquid crystal display panel which can be used for a liquid crystaldisplay device described in this embodiment mode has a structure inwhich a liquid crystal material is interposed between two substrates.Each of the two substrates is provided with an electrode for controllingan electric field applied to the liquid crystal material. A liquidcrystal material corresponds to a material optical and electricalproperties of which are changed by an electric field externally applied.Accordingly, a liquid crystal panel corresponds to a device in whichdesired optical and electrical properties can be obtained by controllingvoltage applied to the liquid crystal material with use of the electrodeincluded in each of the two substrates. In addition, a plurality ofelectrodes are arranged in a planar manner so that each of theelectrodes corresponds to a pixel, and voltages applied to the pixelsare individually controlled; therefore, a liquid crystal display panelwhich can display a high-definition image can be obtained.

Here, response time of the liquid crystal material due to change in anelectric field depends on a space (a cell gap) between the twosubstrates and a type or the like of the liquid crystal material, and isgenerally several milliseconds to several ten milliseconds. When theamount of change in the electric field is small, the response time ofthe liquid crystal material is further lengthened. This characteristiccauses defects in image display, such as an after image, a phenomenon inwhich traces can be seen, and decrease in contrast when the liquidcrystal panel displays a moving image. In particular, when a half toneis changed into another half tone (when change in the electric field issmall), a degree of the above-described defects become noticeable.

On the other hand, as a particular problem of a liquid crystal panelusing an active matrix method, fluctuation in writing voltage due toconstant charge driving is given. Constant charge driving in thisembodiment mode is described below.

A pixel circuit using an active matrix method includes a switch whichcontrols writing and a capacitor which holds a charge. A method fordriving the pixel circuit using the active matrix method corresponds toa method in which predetermined voltage is written in a pixel circuitwith a switch in an on state, and immediately after that, the switch isturned off and a charge in the pixel circuit is held (a hold state). Atthe time of the hold state, exchange of the charge between inside andoutside of the pixel circuit is not performed (a constant charge). Ingeneral, a period when the switch is in an off state is approximatelyseveral hundreds (the number of scan lines) of times longer than aperiod when the switch is in an on state. Accordingly, it is likely thatthe switch of the pixel circuit is almost always in an off state. Asdescribed above, constant charge driving in this embodiment modecorresponds to a driving method in which a pixel circuit is in a holdstate in almost all periods when a liquid crystal panel is driven.

Next, electrical properties of the liquid crystal material aredescribed. A dielectric constant as well as optical properties of theliquid crystal material are changed when an electric field externallyapplied is changed. That is, when it is considered that each pixel ofthe liquid crystal panel is a capacitor (a liquid crystal element)interposed between two electrodes, the capacitor corresponds to acapacitor, capacitance of which is changed in accordance with appliedvoltage. This phenomenon is called dynamic capacitance.

When a capacitor, the capacitance of which is changed in accordance withapplied voltage in this manner, is driven by the constant chargedriving, the following problem occurs. When capacitance of a liquidcrystal element is changed in a hold state in which a charge is notmoved, applied voltage is also changed. This can be understood from thefact that the amount of charges is constant in a relational expressionof (the amount of charges)=(capacitance)×(applied voltage).

For the above-described reasons, voltage at the time of a hold state ischanged from voltage at the time of writing because constant chargedriving is performed in a liquid crystal panel using an active matrixmethod. Accordingly, change in transmittance of the liquid crystalelement is different from change in transmittance of a liquid crystalelement in a driving method which does not take a hold state. FIGS. 51Ato 51C show this state. FIG. 51A shows an example of controlling voltagewritten in a pixel circuit when time is represented by a horizontal axisand an absolute value of the voltage is represented by a vertical axis.FIG. 51B shows an example of controlling voltage written in the pixelcircuit when time is represented by a horizontal axis and the voltage isrepresented by a vertical axis. FIG. 51C shows change in transmittanceof the liquid crystal element over time in the case where the voltageshown in FIG. 51A or 51B is written in the pixel circuit when time isrepresented by a horizontal axis and transmittance of the liquid crystalelement is represented by a vertical axis. In each of FIGS. 51A to 51C,a period F indicates a period for rewriting the voltage, and time forrewriting the voltage is denoted by t₁, t₂, t₃, t₄, and the like.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of 0and corresponds to |V₂| in rewriting at the time of t₁, t₂, t₃, t₄, andthe like (see FIG. 51A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 51B). Since direct voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note that a period of switching thepolarity (an inversion period) may be the same as a period of rewritingvoltage. In this case, generation of a flicker caused by inversiondriving can be reduced because the inversion period is short. Further,the inversion period may be a period which is integral times the periodof rewriting voltage. In this case, power consumption can be reducedbecause the inversion period is long and frequency of writing voltagecan be decreased by changing the polarity.

FIG. 51C shows change in transmittance of the liquid crystal elementover time when voltage as shown in FIG. 51A or 51B is applied to theliquid crystal element. Here, the voltage |V₁| is applied to the liquidcrystal element, and transmittance of the liquid crystal element afterenough time passes corresponds to TR₁. Similarly, the voltage |V₂| isapplied to the liquid crystal element, and transmittance of the liquidcrystal element after enough time passes corresponds to TR₂. When thevoltage applied to the liquid crystal element is changed from |V₁| to|V₂| at the time of t₁, transmittance of the liquid crystal element doesnot immediately become TR₂ but slowly changes as shown by a dashed line5461. For example, when the period of rewriting voltage is the same as aframe period (16.7 milliseconds) of an image signal of 60 Hz, time forseveral frames is necessary until transmittance is changed to TR₂.

Note that smooth change in transmittance over time as shown in thedashed line 5461 corresponds to change in transmittance over time whenthe voltage |V₂| is accurately applied to the liquid crystal element. Inan actual liquid crystal panel, for example, in a liquid crystal panelusing an active matrix method, transmittance of the liquid crystalelement does not changed over time as shown by the dashed line 5461 butgradually changes over time as shown by a solid line 5462. This isbecause voltage at the time of a hold state is changed from voltage atthe time of writing due to constant charge driving, and it is impossibleto reach intended voltage only by one writing. Accordingly, the responsetime of transmittance of the liquid crystal element becomes furtherlonger than original response time (the dashed line 5461) in appearance,so that defects when an image is displayed, such as an after image, aphenomenon in which traces can be seen, or decrease in contrastnoticeably occur.

By using overdriving, it is possible to solve a phenomenon in which theresponse time in appearance becomes further longer because of shortageof writing by dynamic capacitance and constant charge driving as well aslength of the original response time of the liquid crystal element.FIGS. 52A to 52C show this state. FIG. 52A shows an example ofcontrolling voltage written in a pixel circuit when time is representedby a horizontal axis and an absolute value of the voltage is representedby a vertical axis. FIG. 52B shows an example of controlling voltagewritten in the pixel circuit when time is represented by a horizontalaxis and the voltage is represented by a vertical axis. FIG. 52C showschange in transmittance of the liquid crystal element over time in thecase where the voltage shown in FIG. 52A or 52B is written in the pixelcircuit when time is represented by a horizontal axis and transmittanceof the liquid crystal element is represented by a vertical axis. In eachof FIGS. 52A to 52C, a period F indicates a period for rewriting thevoltage, and time for rewriting the voltage is denoted by t₁, t₂, t₃,t₄, and the like.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of0, corresponds to |V₃| in rewriting at the time of t₁, and correspondsto |V₂| in rewriting at the time of t₂, t₃, t₄, and the like (see FIG.52A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 52B). Since direct voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note that a period of switching thepolarity (an inversion period) may be the same as a period of rewritingvoltage. In this case, generation of a flicker caused by inversiondriving can be reduced because the inversion period is short. Further,the inversion period may be a period which is integral times the periodof rewriting voltage. In this case, power consumption can be reducedbecause the inversion period is long and frequency of writing voltagecan be decreased by changing the polarity.

FIG. 52C shows change in transmittance of the liquid crystal elementover time when voltage as shown in FIG. 52A or 52B is applied to theliquid crystal element. Here, the voltage |V₁| is applied to the liquidcrystal element and transmittance of the liquid crystal element afterenough time passes corresponds to TR₁. Similarly, the voltage |V₂| isapplied to the liquid crystal element and transmittance of the liquidcrystal element after enough time passes corresponds to TR₂. Similarly,the voltage |V₃| is applied to the liquid crystal element andtransmittance of the liquid crystal element after enough time passescorresponds to TR₃. When the voltage applied to the liquid crystalelement is changed from |V₁| to |V₃| at the time of t₁, transmittance ofthe liquid crystal element is tried to be changed to TR₃ for severalframes as shown by a dashed line 5471. However, application of thevoltage |V₃| is terminated at the time of t₂, and the voltage |V₂| isapplied after the time of t₂. Therefore, transmittance of the liquidcrystal element does not become as shown by the dashed line 5471 butbecomes as shown by a solid line 5472. It is preferable that a value ofthe voltage |V₃| be set so that transmittance is approximately TR₂ atthe time of t₂. Here, the voltage |V₃| is also referred to asoverdriving voltage.

That is, the response time of the liquid crystal element can becontrolled to some extent by changing |V₃|, which is the overdrivingvoltage. This is because the response time of the liquid crystal elementis changed by the strength of an electric field. Specifically, theresponse time of the liquid crystal element becomes shorter as theelectric field is stronger, and the response time of the liquid crystalelement becomes longer as the electric field is weaker.

It is preferable that |V₃|, which is the overdriving voltage, be changedin accordance with the amount of change in the voltage, that is, thevoltage |V₁| and the voltage |V₂| which provide intended transmittanceTR₁ and TR₂. This is because appropriate response time can be alwaysobtained by changing |V₃|, which is the overdriving voltage, inaccordance with change in the response time of the liquid crystalelement even when the response time of the liquid crystal element ischanged by the amount of change in the voltage.

It is preferable that IVA which is the overdriving voltage, be changeddepending on a mode of the liquid crystal element, such as a TN mode, aVA mode, an IPS mode, or an OCB mode. This is because appropriateresponse time can be always obtained by changing |V₃|, which is theoverdriving voltage, in accordance with change in the response time ofthe liquid crystal element even when the response time of the liquidcrystal element is changed depending on the mode of the liquid crystalelement.

Note that the voltage rewriting period F may be the same as a frameperiod of an input signal. In this case, a liquid crystal display devicewith low manufacturing cost can be obtained since a peripheral drivercircuit of the liquid crystal display device can be simplified.

Note that the voltage rewriting period F may be shorter than the frameperiod of the input signal. For example, the voltage rewriting period Fmay be one half the frame period of the input signal, or one third orless the frame period of the input signal. It is effective to combinethis method with a measure against deterioration in quality of a movingimage caused by hold driving of the liquid crystal display device, suchas black data insertion driving, backlight blinking, backlight scanning,or intermediate image insertion driving by motion compensation. That is,since required response time of the liquid crystal element is short inthe measure against deterioration in quality of a moving image caused byhold driving of the liquid crystal display device, the response time ofthe liquid crystal element can be relatively shortened easily by usingthe overdriving method described in this embodiment mode. Although theresponse time of the liquid crystal element can be shortened by a cellgap, a liquid crystal material, a mode of the liquid crystal element, orthe like, it is technically difficult to shorten the response time ofthe liquid crystal element. Therefore, it is very important to use amethod for shortening the response time of the liquid crystal element bya driving method, such as overdriving.

Note also that the voltage rewriting period F may be longer than theframe period of the input signal. For example, the voltage rewritingperiod F may be twice the frame period of the input signal, or threetimes or more the frame period of the input signal. It is effective tocombine this method with a means (a circuit) which determines whethervoltage is not rewritten for a long period or not. That is, when thevoltage is not rewritten for a long period, an operation of the circuitcan be stopped during a period where no voltage is rewritten withoutperforming a rewriting operation of the voltage. Thus, a liquid crystaldisplay device with low power consumption can be obtained.

Next, a specific method for changing the overdriving voltage |V₃| inaccordance with the voltage |V₁| and the voltage |V₂|, which provideintended transmittance TR₁ and TR₂, is described.

Since an overdriving circuit corresponds to a circuit for appropriatelycontrolling the overdriving voltage |V₃| in accordance with the voltage|V₁| and the voltage |V₂|, which provide intended transmittance TR₁ andTR₂, signals input to the overdriving circuit are a signal related tothe voltage |V₁|, which provides intended transmittance TR₁, and asignal related to the voltage |V₂|, which provides intendedtransmittance TR₂; and a signal output from the overdriving circuit is asignal related to the overdriving voltage |V₃|. Here, each of thesesignals may have an analog voltage value such as the voltage (|V₁|,|V₂|, or |V₃|) applied to the liquid crystal element or may be a digitalsignal for supplying the voltage applied to the liquid crystal element.Here, the signal related to the overdriving circuit is described as adigital signal.

First, a general structure of the overdriving circuit is described withreference to FIG. 48A. Here, input image signals 5401 a and 5401 b areused as signals for controlling the overdriving voltage. As a result ofprocessing these signals, an output image signal 5404 is to be output asa signal which provides the overdriving voltage.

Since the voltage ″V₁| and the voltage |V₂|, which provide intendedtransmittance TR₁ and TR₂, are image signals in adjacent frames, it ispreferable that the input image signals 5401 a and 5401 b be also imagesignals in adjacent frames. In order to obtain such signals, the inputimage signal 5401 a is input to a delay circuit 5402 in FIG. 48A, and asignal which is consequently output can be used as the input imagesignal 5401 b. An example of the delay circuit 5402 includes a memory.That is, the input image signal 5401 a is stored in the memory in orderto delay the input image signal 5401 a for one frame, and at the sametime, a signal stored in the previous frame is extracted from the memoryas the input image signal 5401 b, and the input image signals 5401 a and5401 b are simultaneously input to a correction circuit 5403. Thus, theimage signals in adjacent frames can be handled. By inputting the imagesignals in adjacent frames to the correction circuit 5403, the outputimage signal 5404 can be obtained. Note that when a memory is used asthe delay circuit 5402, a memory having capacity for storing an imagesignal for one frame in order to delay the input image signal 5401 a forone frame (i.e., a frame memory) can be obtained. Thus, the memory canhave a function as a delay circuit without causing excess and deficiencyof memory capacity.

Next, the delay circuit 5402 formed mainly for reducing memory capacityis described. Since memory capacity can be reduced by using such acircuit as the delay circuit 5402, manufacturing cost can be reduced.

Specifically, a delay circuit as shown in FIG. 48B can be used as thedelay circuit 5402 having such characteristics. The delay circuit shownin FIG. 48B includes an encoder 5405, a memory 5406, and a decoder 5407.

Operations of the delay circuit 5402 shown in FIG. 48B are as follows.First, compression processing is performed by the encoder 5405 beforethe input image signal 5401 a is stored in the memory 5406. Thus, thesize of data to be stored in the memory 5406 can be reduced.Accordingly, memory capacity can be reduced, and manufacturing cost canbe reduced. Then, a compressed image signal is transferred to thedecoder 5407 and extension processing is performed here. Thus, thesignal which has been compressed by the encoder 5405 can be restored.Here, compression and extension processing which is performed by theencoder 5405 and the decoder 5407 may be reversible processing.Accordingly, since the image signal does not deteriorate even aftercompression and extension processing is performed, memory capacity canbe reduced without causing deterioration of quality of an image, whichis finally displayed on a device. Alternatively, compression andextension processing which is performed by the encoder 5405 and thedecoder 5407 may be non-reversible processing. Accordingly, since thesize of data of the compressed image signal can be made extremely small,memory capacity can be significantly reduced.

As a method for reducing memory capacity, various methods can be used aswell as the above-described method. For example, a method in which colorinformation included in an image signal is reduced (e.g., tone reductionfrom 260 thousand colors to 65 thousand colors is performed) or theamount of data is reduced (resolution is reduced) without performingimage compression by an encoder can be used.

Next, specific examples of the correction circuit 5403 are describedwith reference to FIGS. 48C to 44E. The correction circuit 5403corresponds to a circuit for outputting an output image signal of acertain value from two input image signals. Here, when a relationbetween the two input image signals and the output image signal isnon-linear and it is difficult to calculate the relation by simpleoperation, a look up table (LUT) may be used as the correction circuit5403. Since the relation between the two input image signals and theoutput image signal is calculated in advance by measurement in a LUT,the output image signal corresponding to the two input image signals canbe calculated only by seeing the LUT (see FIG. 48C). By using a LUT 5408as the correction circuit 5403, the correction circuit 5403 can berealized without complicated circuit design or the like.

Since the LUT is one of memories, it is preferable to reduce memorycapacity as much as possible in order to reduce manufacturing cost. Asan example of the correction circuit 5403 for realizing reduction inmemory capacity, a circuit shown in FIG. 48D can be considered. Thecorrection circuit 5403 shown in FIG. 48D includes a LUT 5409 and anadder 5410. Difference data between the input image signal 5401 a andthe output image signal 5404 to be output is stored in the LUT 5409.That is, corresponding difference data from the input image signal 5401a and the input image signal 5401 b is extracted from the LUT 5409, andthe extracted difference data and the input image signal 5401 a areadded by the adder 5410, so that the output image signal 5404 can beobtained. Note that when data stored in the LUT 5409 is difference data,memory capacity of the LUT can be reduced. This is because the size ofdifference data is smaller than that of the output image signal 5404 asit is, so that memory capacity necessary for the LUT 5409 can bereduced.

In addition, when the output image signal can be calculated by simpleoperation such as four arithmetic operations of the two input imagesignals, the correction circuit 5403 can be realized by combination ofsimple circuits such as an adder, a subtractor, and a multiplier.Accordingly, it is not necessary to use the LUT, and manufacturing costcan be significantly reduced. As such a circuit, a circuit shown in FIG.48E can be considered. The correction circuit 5403 shown in FIG. 48Eincludes a subtractor 5411, a multiplier 5412, and an adder 5413. First,difference between the input image signal 5401 a and the input imagesignal 5401 b is calculated by the subtractor 5411. After that, adifferential value is multiplied by an appropriate coefficient by usingthe multiplier 5412. Then, the differential value multiplied by theappropriate coefficient is added to the input image signal 5401 a by theadder 5413; thus, the output image signal 5404 can be obtained. By usingsuch a circuit, it is not necessary to use the LUT. Therefore,manufacturing cost can be significantly reduced.

By using the correction circuit 5403 shown in FIG. 48E under a certaincondition, inappropriate output of the output image signal 5404 can beprevented. The condition is that the output image signal 5404 applyingthe overdriving voltage and a differential value between the input imagesignals 5401 a and 5401 b have linearity. The slope of this linearity isa coefficient to be multiplied by the multiplier 5412. That is, it ispreferable that the correction circuit 5403 in FIG. 48E be used for aliquid crystal element having such properties. As a liquid crystalelement having such properties, an IPS mode liquid crystal element inwhich response time has little gray-scale dependency is considered. Forexample, when the correction circuit 5403 shown in FIG. 48E is used foran IPS mode liquid crystal element in this manner, manufacturing costcan be significantly reduced and an overdriving circuit which canprevent output of the inappropriate output image signal 5404 can beobtained.

Note that operations which are similar to those of the circuit shown inFIGS. 48A to 48E may be realized by software processing. As the memoryused for the delay circuit, another memory included in the liquidcrystal display device, a memory included in a device which transfers animage displayed on the liquid crystal display device (e.g., a video cardor the like included in a personal computer or a device similar to thepersonal computer), or the like can be used. Accordingly, not only canmanufacturing cost be reduced, intensity of overdriving, availability,or the like can be selected in accordance with user's preference.

Next, driving which controls a potential of a common line is describedwith reference to FIGS. 49A and 49B. FIG. 49A shows a plurality of pixelcircuits in which one common line is provided with respect to one scanline in a display device using a display element which has capacitiveproperties, such as a liquid crystal element. Each of the pixel circuitsshown in FIG. 49A includes a transistor 5421, an auxiliary capacitor5422, a display element 5423, a video signal line 5424, a scan line5425, and a common line 5426.

A gate electrode of the transistor 5421 is electrically connected to thescan line 5425. One of a source electrode and a drain electrode of thetransistor 5421 is electrically connected to the video signal line 5424.The other of the source electrode and the drain electrode of thetransistor 5421 is electrically connected to one electrode of theauxiliary capacitor 5422 and one electrode of the display element 5423.The other electrode of the auxiliary capacitor 5422 is electricallyconnected to the common line 5426.

First, in each pixel selected by the scan line 5425, voltagecorresponding to a video signal is applied to the display element 5423and the auxiliary capacitor 5422 through the video signal line 5424since the transistor 5421 is turned on. At this time, when the videosignal is a signal which makes all of pixels connected to the commonline 5426 display a minimum gray scale or a maximum gray scale, it isnot necessary that the video signal be written in each of the pixelsthrough the video signal line 5424. Voltage applied to the displayelement 5423 can be changed by changing a potential of the common line5426 instead of writing the video signal through the video signal line5424.

Next, FIG. 49B shows a plurality of pixel circuits in which two commonlines are provided with respect to one scan line in a display deviceusing a display element which has capacitive properties, such as aliquid crystal element. Each of the pixel circuits shown in FIG. 49Bincludes a transistor 5431, an auxiliary capacitor 5432, a displayelement 5433, a video signal line 5434, a scan line 5435, a first commonline 5436, and a second common line 5437.

A gate electrode of the transistor 5431 is electrically connected to thescan line 5435. One of a source electrode and a drain electrode of thetransistor 5431 is electrically connected to the video signal line 5434.The other of the source electrode and the drain electrode of thetransistor 5431 is electrically connected to one electrode of theauxiliary capacitor 5432 and one electrode of the display element 5433.The other electrode of the auxiliary capacitor 5432 is electricallyconnected to the first common line 5436. Further, in a pixel which isadjacent to the pixel, the other electrode of the auxiliary capacitor5432 is electrically connected to the second common line 5437.

In the pixel circuits shown in FIG. 49B, the number of pixels which areelectrically connected to one common line is small. Accordingly, bychanging a potential of the first common line 5436 or the second commonline 5437 instead of writing a video signal through the video signalline 5434, frequency of changing voltage applied to the display element5433 is significantly increased. In addition, source inversion drivingor dot inversion driving can be performed. By performing sourceinversion driving or dot inversion driving, reliability of the elementcan be improved and a flicker can be suppressed.

Next, a scanning backlight is described with reference to FIGS. 50A to50C. FIG. 50A shows a scanning backlight in which cold cathodefluorescent lamps are arranged. The scanning backlight shown in FIG. 50Aincludes a diffusion plate 5441 and N pieces of cold cathode fluorescentlamps 5442-1 to 5442-N. The N pieces of the cold cathode fluorescentlamps 5442-1 to 5442-N are arranged on the back side of the diffusionplate 5441, so that the N pieces of the cold cathode fluorescent lamps5442-1 to 5442-N can be scanned while luminance thereof is changed.

Change in luminance of each cold cathode fluorescent lamp in scanning isdescribed with reference to FIG. 50C. First, luminance of the coldcathode fluorescent lamp 5442-1 is changed for a certain period. Afterthat, luminance of the cold cathode fluorescent lamp 5442-2 which isprovided adjacent to the cold cathode fluorescent lamp 5442-1 is changedfor the same period. In this manner, luminance is changed sequentiallyfrom the cold cathode fluorescent lamps 5442-1 to 5442-N. Note thatalthough luminance which is changed for a certain period is set to belower than original luminance in FIG. 50C, it may be higher thanoriginal luminance. In addition, although scanning is performed from thecold cathode fluorescent lamps 5442-1 to 5442-N, scanning may beperformed from the cold cathode fluorescent lamps 5442-N to 5442-1,which is in a reversed order.

By performing driving as in FIG. 50C, average luminance of the backlightcan be decreased. Therefore, power consumption of the backlight, whichmainly takes up power consumption of the liquid crystal display device,can be reduced.

Note that an LED may be used as a light source of the scanningbacklight. FIG. 50B shows the scanning backlight in that case. Thescanning backlight in FIG. 50B includes a diffusion plate 5451 and lightsources 5452-1 to 5452-N, in each of which LEDs are arranged. When theLED is used as the light source of the scanning backlight, it isadvantageous in that the backlight can be thin and lightweight and thata color reproduction area can be widened. Further, since the LEDs whichare arranged in each of the light sources 5452-1 to 5452-N can besimilarly scanned, a dot scanning backlight can also be obtained. Byusing the dot scanning backlight, image quality of a moving image can befurther improved.

When the LED is used as the light source of the backlight, driving canbe performed by changing luminance as shown in FIG. 50C as well.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 13

In this embodiment mode, an example of a display device is described. Inparticular, the case where a display device is optically treated isdescribed.

A rear projection display device 7400 in FIGS. 53A and 53B is providedwith a projector unit 7406, a mirror 7407, and a screen 7401. The rearprojection display device 7400 may also be provided with a speaker 7402and operation switches 7404. The projector unit 7406 is provided at alower portion of a housing 7405 of the rear projection display device7400, and projects incident light for projecting an image based on avideo signal to the mirror 7407. The rear projection display device 7400displays an image projected from a rear surface of the screen 7401.

FIG. 54 shows a front projection display device 7410. The frontprojection display device 7410 is provided with the projector unit 7406and a projection optical system 7411. The projection optical system 7411projects an image to a screen or the like provided at the front.

Hereinafter, a structure of the projector unit 7406 which is applied tothe rear projection display device 7400 in FIGS. 53A and 53B and thefront projection display device 7410 in FIG. 54 is described.

FIG. 55 shows a structure example of the projector unit 7406. Theprojector unit 7406 is provided with a light source unit 7421 and amodulation unit 7424. The light source unit 7421 is provided with alight source optical system 7423 including lenses and a light sourcelamp 7422. The light source lamp 7422 is stored in a housing so thatstray light is not scattered. As the light source lamp 7422, ahigh-pressure mercury lamp or a xenon lamp, for example, which can emita large amount of light is used. The light source optical system 7423 isprovided with an optical lens, a film having a function to polarizelight, a film for adjusting phase difference, an IR film, or the like asappropriate. The light source unit 7421 is provided so that incidentlight is incident on the modulation unit 7424. The modulation unit 7424is provided with a plurality of display panels 7428, a color filter, aretardation plate 7427, a dichroic mirror 7425, a total reflectionmirror 7426, a prism 7429, and a projection optical system 7430. Lightemitted from the light source unit 7421 is split into a plurality ofoptical paths by the dichroic mirror 7425.

Each optical path is provided with the display panel 7428 and a colorfilter which transmits light with a predetermined wavelength orwavelength range. The transmissive display panel 7428 modulatestransmitted light based on a video signal. Light of each colortransmitted through the display panel 7428 is incident on the prism7429, and an image is displayed on the screen through the projectionoptical system 7430. Note that a Fresnel lens may be provided betweenthe mirror and the screen. Projected light which is projected by theprojector unit 7406 and reflected by the mirror is converted intogenerally parallel light by the Fresnel lens to be projected on thescreen. Displacement between a chief ray and an optical axis of theparallel light is preferably ±10° or less, and more preferably, ±5° orless.

The projector unit 7406 shown in FIG. 56 is provided with reflectivedisplay panels 7447, 7448, and 7449.

The projector unit 7406 in FIG. 56 is provided with the light sourceunit 7421 and a modulation unit 7440. The light source unit 7421 mayhave a structure similar to FIG. 55. Light from the light source unit7421 is split into a plurality of optical paths by dichroic mirrors 7441and 7442 and a total reflection mirror 7443 to be incident onpolarization beam splitters 7444, 7445, and 7446. The polarization beamsplitters 7444, 7445, and 7446 are provided corresponding to thereflective display panels 7447, 7448, and 7449 which correspond torespective colors. The reflective display panels 7447, 7448, and 7449modulate reflected light based on a video signal. Light of each color,which are reflected by the reflective display panels 7447, 7448, and7449, is incident on the prism 7450 to be composed, and projectedthrough a projection optical system 7451.

Among light emitted from the light source unit 7421, only light in awavelength region of red is transmitted through the dichroic mirror 7441and light in wavelength regions of green and blue is reflected by thedichroic mirror 7441. Further, only the light in the wavelength regionof green is reflected by the dichroic mirror 7442. The light in thewavelength region of red, which is transmitted through the dichroicmirror 7441, is reflected by the total reflection mirror 7443 andincident on the polarization beam splitter 7444. The light in thewavelength region of blue is incident on the polarization beam splitter7445. The light in the wavelength region of green is incident on thepolarization beam splitter 7446. The polarization beam splitters 7444,7445, and 7446 have a function to split incident light into P-polarizedlight and S-polarized light and a function to transmit only P-polarizedlight. The reflective display panels 7447, 7448, and 7449 polarizeincident light based on a video signal.

Only the S-polarized light corresponding to each color is incident onthe reflective display panels 7447, 7448, and 7449 corresponding to eachcolor. Note that the reflective display panels 7447, 7448, and 7449 maybe liquid crystal panels. In this case, the liquid crystal paneloperates in an electrically controlled birefringence (ECB) mode. Liquidcrystal molecules are vertically aligned at an angle to a substrate.Accordingly, in the reflective display panels 7447, 7448, and 7449, whena pixel is turned off, display molecules are aligned not to change apolarization state of incident light so as to reflect the incidentlight. When the pixel is turned on, alignment of the display moleculesis changed, and the polarization state of the incident light is changed.

The projector unit 7406 in FIG. 56 can be applied to the rear projectiondisplay device 7400 in FIGS. 53A and 53B and the front projectiondisplay device 7410 in FIG. 54.

FIGS. 57A to 57C each show a single-panel type projector unit. Theprojector unit 7406 shown in FIG. 57A is provided with the light sourceunit 7421, a display panel 7467, a projection optical system 7471, and aretardation plate 7464. The projection optical system 7471 includes oneor a plurality of lenses. The display panel 7467 may be provided with acolor filter.

FIG. 57B shows a structure of the projector unit 7406 operating in afield sequential mode. A field sequential mode corresponds to a mode inwhich color display is performed by light of respective colors such asred, green, and blue sequentially incident on a display panel with atime lag, without a color filter. A high-definition image can bedisplayed particularly by combination with a display panel withhigh-speed response to change in input signal. The projector unit 7406in FIG. 57B is provided with a rotating color filter plate 7465including a plurality of color filters with red, green, blue, or thelike between the light source unit 7421 and a display panel 7468.

FIG. 57C shows a structure of the projector unit 7406 with a colorseparation method using a micro lens, as a color display method. Thismethod corresponds to a method in which color display is realized byproviding a micro lens array 7466 on the side of a display panel 7469,on which light is incident, and light of each color is emitted from eachdirection. The projector unit 7406 employing this method has little lossof light due to a color filter, so that light from the light source unit7421 can be efficiently utilized. The projector unit 7406 in FIG. 57C isprovided with dichroic mirrors 7461, 7462, and 7463 so that light ofeach color is emitted to the display panel 7469 from each direction.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 14

In this embodiment mode, examples of electronic devices are described.

FIG. 58 shows a display panel module combining a display panel 9601 anda circuit board 9605. The display panel 9601 includes a pixel portion9602, a scan line driver circuit 9603, and a signal line driver circuit9604. The circuit board 9605 is provided with a control circuit 9606, asignal dividing circuit 9607, and the like, for example. The displaypanel 9601 and the circuit board 9605 are connected to each other by aconnection wiring 9608. An FPC or the like can be used as the connectionwiring.

FIG. 59 is a block diagram showing a main structure of a televisionreceiver. A tuner 9611 receives a video signal and an audio signal. Thevideo signals are processed by an video signal amplifier circuit 9612; avideo signal processing circuit 9613 which converts a signal output fromthe video signal amplifier circuit 9612 into a color signalcorresponding to each color of red, green, and blue; and a controlcircuit 9622 which converts the video signal into the inputspecification of a driver circuit. The control circuit 9622 outputs asignal to each of a scan line driver circuit 9624 and a signal linedriver circuit 9614. The scan line driver circuit 9624 and the signalline driver circuit 9614 drive a display panel 9621. When performingdigital drive, a structure may be employed in which a signal dividingcircuit 9623 is provided on the signal line side so that an inputdigital signal is divided into m signals (m is a positive integer) to besupplied.

Among the signals received by the tuner 9611, an audio signal istransmitted to an audio signal amplifier circuit 9615, and an outputthereof is supplied to a speaker 9617 through an audio signal processingcircuit 9616. A control circuit 9618 receives control information onreceiving station (receiving frequency) and volume from an input portion9619 and transmits signals to the tuner 9611 or the audio signalprocessing circuit 9616.

FIG. 60A shows a television receiver incorporated with a display panelmodule, which is different from FIG. 59. In FIG. 60A, a display screen9632 incorporated in a housing 9631 is formed using the display panelmodule. Note that speakers 9633, input means (an operation key 9634, aconnection terminal 9635, a sensor 9636 (having a function to measurepower, displacement, position, speed, acceleration, angular velocity,the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 9637), and the like may be provided as appropriate.

FIG. 60B shows a television receiver in which only a display can becarried wirelessly. The television receiver is provided with a displayportion 9643, a speaker portion 9647, input means (an operation key9646, a connection terminal 9648, a sensor 9649 (having a function tomeasure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 9641), and the like as appropriate. A battery and a signalreceiver are incorporated in a housing 9642. The battery drives thedisplay portion 9643, the speaker portion 9647, the sensor 9649, and themicrophone 9641. The battery can be repeatedly charged by a charger9640. The charger 9640 can transmit and receive a video signal andtransmit the video signal to the signal receiver of the display. Thedevice in FIG. 60B is controlled by the operation key 9646.Alternatively, the device in FIG. 60B can transmit a signal to thecharger 9640 by operating the operation key 9646. That is, the devicemay be an image and audio interactive communication device. Furtheralternatively, by operating the operation key 9646, the device in FIG.60B may transmit a signal to the charger 9640 and another electronicdevice is made to receive a signal which can be transmitted from thecharger 9640; thus, the device in FIG. 60B can control communication ofanother electronic device. That is, the device may be a general-purposeremote control device. Note that the contents (or part thereof)described in each drawing of this embodiment mode can be applied to thedisplay portion 9643.

Next, a structure example of a mobile phone is described with referenceto FIG. 61.

A display panel 9662 is detachably incorporated in a housing 9650. Theshape and size of the housing 9650 can be changed as appropriate inaccordance with the size of the display panel 9662. The housing 9650which fixes the display panel 9662 is fitted in a printed wiring board9651 to be assembled as a module.

The display panel 9662 is connected to the printed wiring board 9651through an FPC 9663. The printed wiring board 9651 is provided with aspeaker 9652, a microphone 9653, a transmitting/receiving circuit 9654,a signal processing circuit 9655 including a CPU, a controller, and thelike, and a sensor 9661 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray). Such a module, an operation key9656, a battery 9657, and an antenna 9660 are combined and stored in ahousing 9659. A pixel portion of the display panel 9662 is provided tobe seen from an opening window formed in the housing 9659.

In the display panel 9662, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed over the same substrate byusing transistors, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed over an IC chip. Then, the IC chip may bemounted on the display panel 9662 by COG (Chip On Glass). Alternatively,the IC chip may be connected to a glass substrate by using TAB (TapeAutomated Bonding) or a printed wiring board. With such a structure,power consumption of a display device can be reduced and operation timeof the mobile phone per charge can be extended. Further, reduction incost of the mobile phone can be realized.

The mobile phone in FIG. 61 has various functions such as, but notlimited to, a function to display various kinds of information (e.g., astill image, a moving image, and a text image); a function to display acalendar, a date, the time, and the like on a display portion; afunction to operate or edit the information displaying on the displayportion; a function to control processing by various kinds of software(programs); a function of wireless communication; a function tocommunicate with another mobile phone, a fixed phone, or an audiocommunication device by using the wireless communication function; afunction to connect with various computer networks by using the wirelesscommunication function; a function to transmit or receive various kindsof data by using the wireless communication function; a function tooperate a vibrator in accordance with incoming call, reception of data,or an alarm; and a function to generate a sound in accordance withincoming call, reception of data, or an alarm.

FIG. 62A shows a display, which includes a housing 9671, a support base9672, a display portion 9673, a speaker 9677, an LED lamp 9679, inputmeans (a connection terminal 9674, a sensor 9675 (having a function tomeasure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), a microphone9676, and an operation key 9678), and the like. The display in FIG. 62Acan have various functions such as, but not limited to, a function todisplay various kinds of information (e.g., a still image, a movingimage, and a text image) on the display portion.

FIG. 62B shows a camera, which includes a main body 9691, a displayportion 9692, a shutter button 9696, a speaker 9700, an LED lamp 9701,input means (an image receiving portion 9693, operation keys 9694, anexternal connection port 9695, a connection terminal 9697, a sensor 9698(having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), and a microphone 9699), and the like. The camera in FIG.62B can have various functions such as, but not limited to, a functionto photograph a still image and a moving image; a function toautomatically adjust the photographed image (the still image or themoving image); a function to store the photographed image in a recordingmedium (provided externally or incorporated in the camera); and afunction to display the photographed image on the display portion.

FIG. 62C shows a computer, which includes a main body 9711, a housing9712, a display portion 9713, a speaker 9720, an LED lamp 9721, areader/writer 9722, input means (a keyboard 9714, an external connectionport 9715, a pointing device 9716, a connection terminal 9717, a sensor9718 (having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), and a microphone 9719), and the like. The computer inFIG. 62C can have various functions such as, but not limited to, afunction to display various kinds of information (e.g., a still image, amoving image, and a text image) on the display portion; a function tocontrol processing by various kinds of software (programs); acommunication function such as wireless communication or wirecommunication; a function to connect with various computer networks byusing the communication function; and a function to transmit or receivevarious kinds of data by using the communication function.

FIG. 69A shows a mobile computer, which includes a main body 9791, adisplay portion 9792, a switch 9793, a speaker 9799, an LED lamp 9800,input means (operation keys 9794, an infrared port 9795, a connectionterminal 9796, a sensor 9797 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 9798), and thelike. The mobile computer in FIG. 69A can have various functions suchas, but not limited to, a function to display various kinds ofinformation (e.g., a still image, a moving image, and a text image) onthe display portion; a touch panel function provided on the displayportion; a function to display a calendar, a date, the time, and thelike on the display portion; a function to control processing by variouskinds of software (programs); a function of wireless communication; afunction to connect with various computer networks by using the wirelesscommunication function; and a function to transmit or receive variouskinds of data by using the wireless communication function.

FIG. 69B shows a portable image reproducing device having a recordingmedium (e.g., a DVD reproducing device), which includes a main body9811, a housing 9812, a display portion A 9813, a display portion B9814, a speaker portion 9817, an LED lamp 9821, input means (a recordingmedium reading portion 9815 (a recording medium thereof is a DVD or thelike), operation keys 9816, a connection terminal 9818, a sensor 9819(having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), and a microphone 9820), and the like. The display portionA 9813 can mainly display image information, and the display portion B9814 can mainly display text information.

FIG. 69C shows a goggle-type display, which includes a main body 9831, adisplay portion 9832, an earphone 9833, a support portion 9834, an LEDlamp 9839, a speaker 9838, input means (a connection terminal 9835, asensor 9836 (having a function to measure power, displacement, position,speed, acceleration, angular velocity, the number of rotations,distance, light, liquid, magnetism, temperature, a chemical substance,sound, time, hardness, an electric field, current, voltage, electricpower, radiation, a flow rate, humidity, gradient, oscillation, smell,or infrared ray), and a microphone 9837), and the like. The goggle-typedisplay in FIG. 69C can have various functions such as, but not limitedto, a function to display an externally obtained image (e.g., a stillimage, a moving image, and a text image) on the display portion.

FIG. 70A shows a portable game machine, which includes a housing 9851, adisplay portion 9852, a speaker portion 9853, a recording medium insertportion 9855, an LED lamp 9859, input means (an operation key 9854, aconnection terminal 9856, a sensor 9857 (having a function to measurepower, displacement, position, speed, acceleration, angular velocity,the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 9858), and the like. The portable game machine in FIG. 70Acan have various functions such as, but not limited to, a function toread a program or data stored in the recording medium to display on thedisplay portion; and a function to share information by wirelesscommunication with another portable game machine.

FIG. 70B shows a digital camera having a television reception function,which includes a housing 9861, a display portion 9862, a speaker 9864, ashutter button 9865, an LED lamp 9871, input means (an operation key9863, an image receiving portion 9866, an antenna 9867, a connectionterminal 9868, a sensor 9869 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 9870), and thelike. The digital camera having a television reception function in FIG.70B can have various functions such as, but not limited to, a functionto photograph a still image and a moving image; a function toautomatically adjust the photographed image; a function to obtainvarious kinds of information from the antenna; a function to store thephotographed image or the information obtained from the antenna; and afunction to display the photographed image or the information obtainedfrom the antenna on the display portion.

FIG. 71 shows a portable game machine, which includes a housing 9881, afirst display portion 9882, a second display portion 9883, a speakerportion 9884, a recording medium insert portion 9886, an LED lamp 9890,input means (an operation key 9885, a connection terminal 9887, a sensor9888 (having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), and a microphone 9889), and the like. The portable gamemachine in FIG. 71 can have various functions such as, but not limitedto, a function to read a program or data stored in the recording mediumto display on the display portion; and a function to share informationby wireless communication with another portable game machine.

As shown in FIGS. 62A to 62C, 69A to 69C, 70A, 70B, and 71, eachelectronic device includes a display portion for displaying some kind ofinformation.

Next, application examples of a semiconductor device are described.

FIG. 63 shows an example where a semiconductor device is incorporated ina constructed object. FIG. 63 shows a housing 9730, a display portion9731, a remote control device 9732 which is an operation portion, aspeaker portion 9733, and the like. The semiconductor device isincorporated in the constructed object as a wall-hanging type and can beprovided without requiring a large space.

FIG. 64 shows another example where a semiconductor device isincorporated in a constructed object. A display panel 9741 isincorporated with a prefabricated bath 9742, and a person who takes abath can view the display panel 9741. The display panel 9741 has afunction to display information by an operation by a person who takes abath; and a function to be used as an advertisement or an entertainmentmeans.

Note that the semiconductor device can be provided not only to a sidewall of the prefabricated bath 9742 as shown in FIG. 64, but also tovarious places. For example, the semiconductor device can beincorporated with part of a mirror, a bathtub itself, or the like. Atthis time, a shape of the display panel 9741 may be changed inaccordance with a shape of the mirror or the bathtub.

FIG. 65 shows another example where a semiconductor device isincorporated in a constructed object. A display panel 9752 is bent andattached to a curved surface of a column-shaped object 9751. Here, autility pole is described as the column-shaped object 9751.

The display panel 9752 in FIG. 65 is provided at a position higher thana human viewpoint. When the same images are displayed on the displaypanels 9752 provided in constructed objects which stand together inlarge numbers outdoors, such as utility poles, advertisement can beperformed to a plurality of unspecified viewers. Since it is easy forthe display panel 9752 to display the same images and instantly switchimages by external control, highly efficient information display andadvertisement effect can be obtained. When provided with self-luminousdisplay elements, the display panel 9752 can be effectively used as ahighly visible display medium even at night. When the display panel 9752is provided in the utility pole, a power supply means for the displaypanel 9752 can be easily obtained. In an emergency such as disaster, thedisplay panel 9752 can also be used as a means to transmit correctinformation to victims rapidly.

An example of the display panel 9752 includes a display panel in which aswitching element such as an organic transistor is provided over afilm-shaped substrate and a display element is driven so as to displayan image.

Note that in this embodiment mode, a wall, a column-shaped object, and aprefabricated bath are shown as examples of a constructed object;however, this embodiment mode is not limited thereto, and variousconstructed objects can be provided with a semiconductor device.

Next, examples where a semiconductor device is incorporated with amoving object are described.

FIG. 66 shows an example where a semiconductor device is incorporatedwith a car. A display panel 9761 is incorporated with a car body 9762,and can display an operation of the car body or information input frominside or outside the car body on demand. Note that a navigationfunction may be provided.

The semiconductor device can be provided not only to the car body 9762as shown in FIG. 66, but also to various places. For example, thesemiconductor device can be incorporated with a glass window, a door, asteering wheel, a gear shift, a seat, a rear-view mirror, and the like.At this time, a shape of the display panel 9761 may be changed inaccordance with a shape of an object provided with the semiconductordevice.

FIGS. 67A and 67B show examples where a semiconductor device isincorporated with a train car.

FIG. 67A shows an example where a display panel 9772 is provided inglass of a door 9771 in a train car, which has an advantage comparedwith a conventional advertisement using paper in that labor cost forchanging an advertisement is not necessary. Since the display panel 9772can instantly switch images displaying on a display portion by anexternal signal, images on the display panel can be switched in everytime period when types of passengers on the train are changed, forexample; thus, more effective advertisement effect can be obtained.

FIG. 67B shows an example where the display panels 9772 are provided toa glass window 9773 and a ceiling 9774 as well as the glass of the door9771 in the train car. In this manner, the semiconductor device can beeasily provided to a place where a semiconductor device has beendifficult to be provided conventionally; thus, effective advertisementeffect can be obtained. Further, the semiconductor device can instantlyswitch images displayed on a display portion by an external signal;thus, cost and time for changing an advertisement can be reduced, andmore flexible advertisement management and information transmission canbe realized.

The semiconductor device can be provided not only to the door 9771, theglass window 9773, and the ceiling 9774 as shown in FIG. 67, but also tovarious places. For example, the semiconductor device can beincorporated with a strap, a seat, a handrail, a floor, and the like. Atthis time, a shape of the display panel 9772 may be changed inaccordance with a shape of an object provided with the semiconductordevice.

FIGS. 68A and 68B show an example where a semiconductor device isincorporated with a passenger airplane.

FIG. 68A shows a shape of a display panel 9782 attached to a ceiling9781 above a seat of the passenger airplane when the display panel 9782is used. The display panel 9782 is incorporated with the ceiling 9781using a hinge portion 9783, and a passenger can view the display panel9782 by stretching of the hinge portion 9783. The display panel 9782 hasa function to display information by an operation by the passenger and afunction to be used as an advertisement or an entertainment means. Asshown in FIG. 68B, the hinge portion is bent and the display panel isstored in the ceiling 9781 of the airplane, so that safety in taking-offand landing can be assured. Note that a display element in the displaypanel is lit in an emergency, so that the display panel can also be usedas an information transmission means and an evacuation light.

The semiconductor device can be provided not only to the ceiling 9781 asshown in FIGS. 68A and 68B, but also to various places. For example, thesemiconductor device can be incorporated with a seat, a table attachedto a seat, an armrest, a window, and the like. A large display panelwhich a plurality of people can view may be provided at a wall of anairframe. At this time, a shape of the display panel 9782 may be changedin accordance with a shape of an object provided with the semiconductordevice.

Note that in this embodiment mode, bodies of a train car, a car, and anairplane are shown as a moving object; however, the invention is notlimited thereto, and a semiconductor device can be provided to variousobjects such as a motorcycle, an four-wheel drive car (including a car,a bus, and the like), a train (including a monorail, a railroad car, andthe like), and a vessel. Since the semiconductor device can instantlyswitch images displayed on a display panel in a moving object by anexternal signal, the moving object provided with the semiconductordevice can be used as an advertisement display board for a plurality ofunspecified customers, an information display board in disaster, and thelike.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment 1

In this embodiment, an example where a liquid crystal display device isactually manufactured using the structure in Embodiment Mode 1 isdescribed with reference to FIGS. 14A, 14B, 15A to 15D, 16A to 16C, 17Ato 17C, and 18. Note that this embodiment can be completed by using, asa basic structure, each part of the structures in Embodiment Modes 2 to6 as well as the structure in Embodiment Mode 1.

It is needless to say that a bottom-gate TFT described in EmbodimentMode 2, a structure described in Embodiment Mode 3, in which a pixelelectrode is directly connected to an island-shaped semiconductor film,a connection structure of electrodes described in Embodiment Mode 4, ashape of a pixel electrode described in Embodiment Mode 5, a colorfilter described in Embodiment Mode 6, or the like can be combined withthis embodiment when needed.

FIG. 14A is a top plan view of a liquid crystal display device in thisembodiment, and FIG. 14B is a cross-sectional view thereof. Thisembodiment is an example of a manufacturing method of a liquid crystaldisplay device which has the structure shown in Embodiment Mode 1.Accordingly, the degree of freedom of a space between a common electrode(corresponding to the conductive film 115 in FIG. 1) and a pixelelectrode (corresponding to the pixel electrodes 113 and 114 in FIG. 1)is increased. Since optimal values for an arrangement interval and thewidth of an opening of a pixel electrode are changed depending on adistance between the pixel electrode and the common electrode, the size,the width, and the interval of the opening can be freely set. Further, agradient of an electric field applied between the electrodes can becontrolled, and an electric field parallel to a substrate can be easilyincreased, for example. That is, in a display device using a liquidcrystal, since liquid crystal molecules aligned in parallel to asubstrate (so-called homogeneous alignment) can be controlled in adirection parallel to the substrate, a viewing angle is widened byapplying an optimal electric field.

First, as shown in FIG. 15A, a conductive film 801 having alight-transmitting property is formed over a substrate 800. Thesubstrate 800 is a glass substrate, a quartz substrate, a substrateformed of an insulator such as alumina, a plastic substrate with heatresistance high enough to withstand a processing temperature ofsubsequent steps, a silicon substrate, or a metal plate. Alternatively,the substrate 800 may be a substrate in which an insulating film such asa silicon oxide film or a silicon nitride film is formed on a surface ofmetal such as stainless steel, a semiconductor substrate, or the like.Note that when a plastic substrate is used as the substrate 800, it ispreferable to use a plastic substrate having a relatively high glasstransition point, such as PC (polycarbonate), PES (polyethersulfone),PET (polyethylene terephthalate), or PEN (polyethylene naphthalate).

The conductive film 801 is, for example, an indium tin oxide (ITO) film,an indium tin oxide film containing a Si element, or an film using amaterial (in this specification, also referred to as indium zinc oxide(IZO)) formed using a target in which zinc oxide (ZnO) of 2 to 20 wt %is mixed with indium oxide.

Next, an insulating film 802 is formed as a base film over theconductive film 801 and the substrate 800. The insulating film 802 is,for example, a film in which a silicon oxide film is stacked on asilicon nitride film; however, it may be another insulator (e.g., asilicon oxide film containing nitrogen or a silicon nitride filmcontaining oxygen).

Here, by performing nitriding with high-density plasma on a surface ofthe insulating film 802 formed of the silicon oxide film, the siliconoxide film containing nitrogen, or the like, a silicon nitride film maybe formed on the surface of the insulating film 802.

For example, high-density plasma is generated by using a microwave of2.45 GHz, and has an electron density of 1×10¹¹ to 1×10¹³/cm³, anelectron temperature of 2 eV or less, and an ion energy of 5 eV or less.Such high-density plasma has low kinetic energy of active species, andcan form a film with less plasma damage and fewer defects compared withconventional plasma treatment. A distance from an antenna generating amicrowave to the insulating film 802 is set to 20 to 80 mm, andpreferably 20 to 60 mm.

The surface of the insulating film 802 can be nitrided by performing thehigh-density plasma treatment in a nitrogen atmosphere, such as anatmosphere including nitrogen and rare gas, an atmosphere includingnitrogen, hydrogen, and rare gas, or an atmosphere including ammonia andrare gas.

Since a silicon nitride film can suppress impurity diffusion from thesubstrate 800 and can be formed to be extremely thin by the high-densityplasma treatment, influence of stress on a semiconductor film to beformed thereover can be reduced.

Then, as shown in FIG. 15B, a crystalline semiconductor film (e.g., apolycrystalline silicon film) is formed as a semiconductor film 803 overthe insulating film 802. Examples of a method of forming the crystallinesemiconductor film include a method of directly forming the crystallinesemiconductor film over the insulating film 802, and a method of formingan amorphous semiconductor film over the insulating film 802 and thencrystallizing the amorphous semiconductor film.

As a method for crystallizing the amorphous semiconductor film, a methodof laser light irradiation, a method of thermal crystallization using anelement which promotes crystallization of a semiconductor film (e.g., ametal element such as nickel), or a method of laser light irradiationafter thermal crystallization using an element which promotescrystallization of the semiconductor film can be used. It is needless tosay that a method of thermal crystallization of the amorphoussemiconductor film without using the above-described element can also beused; however, it is limited to the case of a substrate which canwithstand high temperature, such as a quartz substrate or a siliconwafer.

When laser irradiation is used, a continuous wave laser beam (a CW laserbeam) or a pulsed laser beam (a pulse laser beam) can be used. As alaser beam which can be used here, a laser emitted from one or more ofthe following can be used: a gas laser such as an Ar laser, a Kr laser,or an excimer laser; a laser of which a medium is single crystallineYAG, YVO₄, forsterite (Mg₂SiO₄), YAlO₃, or GdVO₄, or polycrystalline(ceramic) YAG, Y₂O₃, YVO₄, YAlO₃, or GdVO₄, added with one or more ofNd, Yb, Cr, Ti, Ho, Er, Tm, and Ta as a dopant; a glass laser; a rubylaser; an alexandrite laser; a Ti:sapphire laser; a copper vapor laser;or a gold vapor laser. Crystals with a large grain size can be obtainedby irradiation with a fundamental wave of such a laser beam or second tofourth harmonics of the fundamental wave. For example, the secondharmonic (532 nm) or the third harmonic (355 nm) of an Nd:YVO4 laser(fundamental wave of 1064 nm) can be used. In this case, a power densityof the laser is needed to be approximately 0.01 to 100 MW/cm²(preferably, 0.1 to 10 MW/cm²). Irradiation is performed with a scanningrate of approximately 10 to 2000 cm/sec.

Note that a laser of which a medium is single crystalline YAG, YVO₄,forsterite (Mg₂SiO₄), YAlO₃, or GdVO₄, or polycrystalline (ceramic) YAG,Y₂O₃, YVO₄, YAlO₃, or GdVO₄ added with one or more of Nd, Yb, Cr, Ti,Ho, Er, Tm, and Ta as a dopant; an Ar ion laser; or a Ti:sapphire lasercan be continuously oscillated, and pulse oscillation thereof can beperformed at a repetition rate of 10 MHz or more by performing Q-switchoperation, mode locking, or the like. When a laser beam is oscillated ata repetition rate of 10 MHz or more, a semiconductor film is irradiatedwith the next pulse while the semiconductor film is melted by the laserand solidified. Accordingly, unlike the case of using a pulsed laserwith a low repetition rate, a solid-liquid interface can be continuouslymoved in the semiconductor film; thus, crystal grains which continuouslygrow in a scanning direction can be obtained.

When ceramic (polycrystal) is used as a medium, the medium can be formedto have a desired shape for a short time and at low cost. When a singlecrystalline is used, a columnar medium with several mm in diameter andseveral tens of mm in length is used. When the ceramic is used, a mediumlarger than the case of using the single crystalline can be formed.

A concentration of a dopant such as Nd or Yb in a medium, which directlycontributes to light emission, cannot be changed largely in either caseof the single crystalline or the polycrystal; thus, there is somelimitation on improvement in output of a laser by increasing theconcentration of the dopant. However, in the case of ceramic, the sizeof the medium can be significantly increased as compared with the caseof the single crystalline; thus, drastic improvement in output of alaser can be realized.

In addition, in the case of ceramic, a medium with a parallelepipedshape or a rectangular parallelepiped shape can be easily formed. When amedium having such a shape is used and oscillated light is made travelin a zigzag manner inside the medium, a path of the oscillated light canbe made long. Therefore, amplification is increased, and a laser beamcan be oscillated at high output. Further, since a cross section of alaser beam emitted from the medium having such a shape has aquadrangular shape, it has an advantage over a circular beam in beingshaped into a linear beam. By shaping a laser beam emitted in such amanner by using an optical system, a linear beam having a length of 1 mmor less on a lateral side and a length of several mm to several m on alongitudinal side can be easily obtained. Furthermore, when a medium isuniformly irradiated with excited light, energy distribution of a linearbeam becomes uniform in a longitudinal direction.

A semiconductor film is irradiated with this linear beam, so that thewhole surface of the semiconductor film can be annealed more uniformly.When uniform annealing is needed from one end to the other end of thelinear beam, ingenuity such as arrangement in which slits are providedin both ends of the linear beam to shield light at a portion whereenergy is attenuated is necessary.

When a semiconductor film is annealed using the thus obtained linearbeam having uniform intensity and an electronic device is formed byusing this semiconductor film, characteristics of the electronic deviceare favorable and uniform.

As the method for crystallizing the amorphous semiconductor film byheating with an element which promotes crystallization of thesemiconductor film, an amorphous semiconductor film (also referred to asan amorphous silicon film) is doped with a metal element which promotescrystallization of the semiconductor film, and then heat treatment isperformed so that the amorphous semiconductor film is crystallized withthe doped region as a nucleus.

Alternatively, an amorphous semiconductor film can be crystallized byperforming irradiation with strong light instead of heat treatment. Inthis case, one or a combination of infrared light, visible light, andultraviolet light can be used. Typically, light emitted from a halogenlamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a highpressure sodium lamp, or a high pressure mercury lamp is used. A lamplight source is lighted for 1 to 60 seconds, preferably 30 to 60seconds, and such lighting is repeated 1 to 10 times, preferably 2 to 6times. The light emission intensity of the lamp light source is decidedas appropriate, so that the semiconductor film is instantaneously heatedup to approximately 600 to 1000° C. Note that when necessary, heattreatment may be performed in order to discharge hydrogen contained inthe amorphous semiconductor film having an amorphous structure beforeirradiation with strong light. Alternatively, crystallization may beperformed by both heat treatment and irradiation with strong light.

After the heat treatment, the crystalline semiconductor film may beirradiated with the laser light in the atmospheric air or an oxygenatmosphere in order to increase the degree of crystallinity (a ratio ofcrystalline components in the whole volume of the film) of thecrystalline semiconductor film and to correct defects which remain incrystalline grains. The laser light can be selected from theaforementioned laser light.

The doped elements are needed to be removed from the crystallinesemiconductor film, and the method is described below.

First, a surface of the crystalline semiconductor film is treated with asolution containing ozone (typically, ozone water), so that a barrierlayer formed of an oxide film (called chemical oxide) having a thicknessof 1 to 10 nm is formed on the surface of the crystalline semiconductorfilm. The barrier layer functions as an etching stopper when only agettering layer is selectively removed in a subsequent step.

Then, a gettering layer containing a rare gas element is formed as agettering site over the barrier layer. Here, a semiconductor filmcontaining a rare gas element is formed as the gettering layer by a CVDmethod or a sputtering method. When the gettering layer is formed, thesputtering conditions are controlled as appropriate so that a rare gaselement is added to the gettering layer. The rare gas element may be oneor more of helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon(Xe).

Note that when the gettering layer is formed by using a source gascontaining phosphorus, which is an impurity element, or by using atarget containing phosphorus, gettering can be performed by utilizingthe coulomb force of phosphorus in addition to gettering using the raregas element. In gettering, a metal element (such as nickel) tends tomove to a region having a high concentration of oxygen; therefore, theconcentration of oxygen contained in the gettering layer is preferablyset at, for example, 5×10¹⁸ cm⁻³ or higher.

Next, the crystalline semiconductor film, the barrier layer, and thegettering layer are subjected to thermal treatment (e.g., heat treatmentor irradiation with strong light), so that the metal element (such asnickel) is gettered; thus, the metal element in the crystallinesemiconductor film is lowered in concentration or removed.

Next, a known etching method is performed using the barrier layer as theetching stopper so that only the gettering layer is selectively removed.After that, the barrier layer formed of an oxide film is removed by, forexample, an etchant containing hydrofluoric acid.

Here, impurity ions may be doped in consideration of thresholdcharacteristics of a TFT to be manufactured.

Next, a photo resist film (not shown) is applied over the semiconductorfilm 803 by a coating method, and is exposed to light and developed. Acoating method includes a spin coating method, a spray method, a screenprinting method, a paint method, or the like. Thus, a resist is formedover the semiconductor film 803. Then, the semiconductor film 803 isetched using the resist as a mask. Thus, island-shaped semiconductorfilms 872, 873, and 874 in which thin film transistors are formed areformed over the insulating film 802.

Then, after surfaces of the island-shaped semiconductor films 872 to 874are cleaned with an etchant containing hydrofluoric acid or the like, agate insulating film 804 having a thickness of 10 to 200 nm is formedover the island-shaped semiconductor films 872 to 874. The gateinsulating film 804 is formed of an insulating film containing siliconas a main component, such as a silicon oxide film, a silicon nitridefilm, a silicon oxide film containing nitrogen, or a silicon nitridefilm containing oxygen. Further, the gate insulating film may have asingle-layer structure or a stacked-layer structure. Note that the gateinsulating film 804 is also formed over the insulating film 802.

After the gate insulating film 804 is formed, gate electrodes 865, 866,867, and 868; an electrode 869; impurity regions 807 a, 807 b, 808 a,808 b, 809 a, 809 b, 810 a, 810 b, 813 a, 813 b, 813 c, 814 a, 814 b,814 c, and 814 d; and channel formation regions 895, 896, and 897 (897 aand 897 b) are formed (see FIG. 15D).

The gate electrode 865 of a TFT 827 includes a lower gate electrode 805a and an upper gate electrode 806 a. The gate electrode 866 of a TFT 829includes a lower gate electrode 805 b and an upper gate electrode 806 b.The gate electrode 867 of a TFT 825 includes a lower gate electrode 805c and an upper gate electrode 806 c, and the gate electrode 868 of theTFT 825 includes a lower gate electrode 805 d and an upper gateelectrode 806 d.

The electrode 869 includes a lower electrode 861 and an upper electrode862.

Each of the impurity regions 807 a and 807 b is a source region or adrain region of the TFT 827. The impurity regions 808 a and 808 b arelow concentration impurity regions of the TFT 827. The channel formationregion 895 is located between the impurity regions 808 a and 808 b.

Each of the impurity regions 809 a and 809 b are a source region or adrain region of the TFT 829. The impurity regions 810 a and 810 b arelow concentration impurity regions of the TFT 829. The channel formationregion 896 is located between the impurity regions 810 a and 810 b.

Each of the impurity regions 813 a and 813 c are a source region or adrain region of the TFT 825. The impurity region 813 b is formed in thesame step as the impurity regions 813 a and 813 c. The impurity regions814 a, 814 b, 814 c, and 814 d are low concentration impurity regions ofthe TFT 825. The channel formation region 897 a is located between theimpurity regions 814 a and 814 b. The channel formation region 897 b islocated between the impurity regions 814 c and 814 d.

In this embodiment, the impurity regions 809 a, 809 b, 810 a, 810 b, 813a to 813 c, and 814 a to 814 d are n-type impurity regions, and includean impurity element imparting n-type conductivity, such as phosphorus(P) or arsenic (As). The impurity regions 809 a, 809 b, and 813 a to 813c are high concentration impurity regions, and they have impurityconcentrations higher than those of the impurity regions 810 a, 810 b,and 814 a to 814 d, which are low concentration impurity regions.

In this embodiment, the impurity regions 807 a, 807 b, 808 a, and 808 bare p-type impurity regions, and include an impurity element impartingp-type conductivity, such as boron (B). The impurity regions 807 a and807 b are high concentration impurity regions, and they have impurityconcentrations higher than those of the impurity regions 808 a and 808b, which are low concentration impurity regions.

That is, the TFTs 829 and 825 are n-channel transistors, and the TFT 827is a p-channel transistor.

A manufacturing method of the gate electrodes 865 to 868 and theelectrode 869 is described below.

After the gate insulating film 804 is formed, the gate insulating film804 is cleaned. Then, a first conductive film and a second conductivefilm are formed in this order over the gate insulating film 804. Forexample, the first conductive film is a tungsten film and the secondconductive film is a tantalum nitride film.

Next, a photo resist film is applied over the second conductive film,and is exposed to light and developed. Thus, a resist is formed over thesecond conductive film. Then, by using the resist as a mask, the firstconductive film and the second conductive film are etched under a firstcondition, and further, the second conductive film is etched under asecond condition. Thus, the lower gate electrode 805 a and the uppergate electrode 806 a are formed over the island-shaped semiconductorfilm 872; the lower gate electrode 805 b and the upper gate electrode806 b are formed over the island-shaped semiconductor film 873; thelower gate electrode 805 c and the upper gate electrode 806 c, and thelower gate electrode 805 d and the upper gate electrode 806 d are formedover the island-shaped semiconductor film 874.

Inclined angles of side surfaces of the lower gate electrodes 805 a to805 d are more moderate than inclined angles of side surfaces of theupper gate electrodes 806 a to 806 d.

In addition, the lower electrode 861 and the upper electrode 862 areformed at the same time.

Thereafter, the photo resist film is removed.

The impurity regions 807 a, 807 b, 808 a, 808 b, 809 a, 809 b, 810 a,810 b, 813 a to 813 c, and 814 a to 814 d may be formed by introducingimpurities in a self-aligned manner by using the gate electrodes 865 to868 as masks, or may be formed by introducing impurities using a resistmask.

Thereafter, an insulating film (not shown) covering almost all surfaceis formed. The insulating film is, for example, a silicon oxide filmformed by a plasma CVD method.

Next, heat treatment is performed on the island-shaped semiconductorfilms 872 to 874 to activate the impurity elements doped therewith. Theheat treatment is performed by a rapid thermal annealing method (RTAmethod) using a lamp light source, irradiation of a YAG laser or anexcimer laser from the back surface, heat treatment using a furnace, ora combination of a plurality of these methods.

By the heat treatment, the impurity elements are activated, andsimultaneously the element (e.g., a metal element such as nickel) whichis used as a catalyst for crystallizing the island-shaped semiconductorfilms 873 and 874 are gettered in the impurity regions 809 a, 809 b, and813 a to 813 c including a high concentration impurity (such asphosphorus), and a nickel concentration mainly in a portion to be thechannel formation regions 896, 897 a, and 897 b in the island-shapedsemiconductor films 873 and 874 is reduced. As a result, thecrystallinity of the channel formation regions is improved. Accordingly,an off-current value of the TFT is reduced and high electronfield-effect mobility can be obtained. Thus, a TFT having favorablecharacteristics can be obtained.

Next, an insulating film 815 is formed over the entire surface includingabove the island-shaped semiconductor films 872 to 874. The insulatingfilm 815 is, for example, a silicon nitride film formed by a plasma CVDmethod.

Then, a planarization film to be an interlayer insulating film 816 isformed over the insulating film 815. As the interlayer insulating film816, a light-transmitting inorganic material (e.g., silicon oxide,silicon nitride, or silicon nitride containing oxygen); a photosensitiveor non-photosensitive organic material (e.g., polyimide, acrylic,polyamide, polyimide amide, resist, or benzocyclobutene); astacked-layer structure thereof; or the like is used. Alternatively, asanother light-transmitting film used for the planarization film, aninsulating film formed of a SiOx film containing an alkyl group obtainedby a coating method, such as an insulating film formed using silicaglass, an alkylsiloxane polymer, an alkylsilsesquioxane polymer, ahydrogen silsesquioxane polymer, a hydrogen alkylsilsesquioxane polymer,or the like can be used. Examples of siloxane-based polymers includecoating insulating film materials such as PSB-K1 and PSB-K31 (product ofToray industries, Inc.) and ZRS-5PH (product of Catalysts & ChemicalsIndustries Co., Ltd.). The interlayer insulating film 816 may be asingle-layer film or a multi-layer film.

Next, a photo resist film (not shown) is applied over the interlayerinsulating film 816, and is exposed to light and developed. Thus, aresist is formed over the interlayer insulating film 816. Then, theinterlayer insulating film 816, the insulating film 815, and the gateinsulating film 804 are etched using the resist as a mask. Accordingly,contact holes 817 a, 817 b, 817 c, 817 d, 817 e, 817 f, 817 g, and 817 hare formed in the interlayer insulating film 816, the insulating film815, and the gate insulating film 804.

The contact hole 817 a is located over the impurity region 807 a. Thecontact hole 817 b is located over the impurity region 807 b. Thecontact hole 817 c is located over the impurity region 809 a. Thecontact hole 817 d is located over the impurity region 809 b. Thecontact hole 817 e is located over the impurity region 813 a. Thecontact hole 817 f is located over the impurity region 813 c. Thecontact hole 817 g is located over the conductive film 801. The contacthole 817 h is located over the electrode 869.

Thereafter, the resist is removed.

Next, as shown in FIG. 16B, a first conductive film 875 is formed in thecontact holes 817 a to 817 h and over the interlayer insulating film816. The first conductive film 875 is a light-transmitting conductivefilm, such as an indium tin oxide film, a film of indium tin oxidecontaining silicon, or a film formed using a target in which zinc oxideof 2 to 20 wt % is mixed with indium oxide. Then, a second conductivefilm 876 is formed over the first conductive film 875. The secondconductive film 876 is, for example, a metal film.

Next, a photo resist film 820 is applied over the second conductive film876. Then, a reticle 840 is provided above the photo resist film 820.The reticle 840 has a structure where semi-transparent films 841 a, 841b, 841 c, 841 d, 841 e, 841 f, and 841 g are formed over a glasssubstrate, and light shielding films 84842 b, 842 c, 842 d, 842 e, 842f, and 842 g are formed over the semi-transparent films 841 a, 841 b,841 c, 841 d, 841 e, 841 f, and 841 g, respectively. Thesemi-transparent film 841 a and the light shielding film 842 a arelocated above the contact hole 817 a. The semi-transparent film 841 band the light shielding film 842 b are located above the contact hole817 b. The semi-transparent film 841 c and the light shielding film 842c are located above the contact hole 817 c. The semi-transparent film841 d and the light shielding film 842 d are located above the contacthole 817 d. The semi-transparent film 841 e and the light shielding film842 e are located above the contact hole 817 e. The semi-transparentfilm 841 f and the light shielding film 842 f are located above thecontact hole 817 f. The semi-transparent film 841 g and the lightshielding film 842 g are located above the contact holes 817 g and 871h.

Next, the photo resist film 820 is exposed to light using the reticle840 as a mask. Thus, the photo resist film 820 is exposed to lightexcept for portions below the light shielding films 842 a to 842 g and alower layer of portions below the semi-transparent films 841 a to 841 g.Note that portions which are not exposed to light are denoted by regions821 a, 821 b, 821 c, 821 d, 821 e, 821 f, and 821 g.

Next, as shown in FIG. 17A, the photo resist film 820 is developed.Thus, portions of the photo resist film 820, which are exposed to light,are removed, and resists 822 a, 822 b, 822 c, 822 d, 822 e, 822 f, and822 g are formed. The resist 822 a is located above the contact hole 817a. The resist 822 b is located above the contact hole 817 b. The resist822 c is located above the contact hole 817 c. The resist 822 d islocated above the contact hole 817 d. The resist 822 e is located abovethe contact hole 817 e. The resist 822 f is located above the contacthole 817 f. The resist 822 g is located above the contact holes 817 gand 817 h.

Then, as shown in FIG. 17B, the first conductive film 875 and the secondconductive film 876 are etched using the resists 822 a to 822 g asmasks. Thus, the first conductive film 818 and the second conductivefilm 819 in regions which are not covered with the resists 822 a to 822g are removed.

Thereafter, the resists 822 a to 822 g are removed.

In such a manner, an electrode 881 having a lower electrode 824 a and anupper electrode 823 a, an electrode 882 having a lower electrode 824 band an upper electrode 823 b, an electrode 883 having a lower electrode824 c and an upper electrode 823 c, an electrode 884 having a lowerelectrode 824 d and an upper electrode 823 d, an electrode 885 having alower electrode 824 e and an upper electrode 823 e, an electrode 886having a lower electrode 824 f and an upper electrode 823 f, and anelectrode 887 having a lower electrode 863 and an upper electrode 864are formed with one resist and one etching treatment.

The electrodes 881 to 887 may be electrically connected to each other byforming an additional wiring or may be formed as wirings. In the lattercase, the electrodes 881 to 887 are described as the wirings 881 to 887.

The electrodes 881, 882, 883, 884, 885, and 886 are electricallyconnected to the impurity regions 807 a, 807 b, 809 a, 809 b, 813 a, and813 c, respectively. The electrode 887 is electrically connected to theconductive film 801 and the electrode 869.

Next, an interlayer insulating film 845 is formed over the interlayerinsulating film 816 and the electrodes 881 to 887 (see FIG. 17C). Theinterlayer insulating film 845 may be formed of a material similar tothat of the interlayer insulating film 816.

Next, a contact hole reaching the electrode 886 is formed in theinterlayer insulating film 845. Then, pixel electrodes 891 (891 a, 891b, 891 c, 891 d, and the like) which are electrically connected to theelectrode 886 through the contact hole are formed (see FIG. 18). Thepixel electrodes 891 is formed of a light-transmitting material, and amaterial similar to that of the conductive film 875 may be used. Grooves892 (892 a, 892 b, 892 c, and the like) are formed in the pixelelectrodes 891. FIGS. 4, 7, 8A to 8D, and 9A to 9D may be used forshapes of the pixel electrodes 891 and the grooves 892.

Thereafter, a first alignment film 826 is formed. In such a manner, anactive matrix substrate is formed.

Note that the TFTs 827 and 829 are formed in a gate signal line drivercircuit 854. The TFTs separately formed are shown in FIG. 14B; however,the electrodes 882 and 883 may be electrically connected so that theTFTs 827 and 829 may be formed as a CMOS circuit.

Further, a first terminal electrode 838 a and a second terminalelectrode 838 b (shown in FIG. 14B) which connect the active matrixsubstrate and the outside are formed.

Thereafter, as shown in a plan view of FIG. 14A and a cross-sectionalview along a line K-L of FIG. 14B, an organic resin film such as anacrylic resin film is formed over the active matrix substrate. Then, theorganic resin film is selectively removed by etching with the use of amask film. Thus, a columnar spacer 833 is formed over the active matrixsubstrate. Next, after a sealing material 834 is formed in a sealingregion 853, a liquid crystal is dropped on the active matrix substrate.Before the liquid crystal is dropped, a protective film may be formedover the sealing material to prevent the sealing material and the liquidcrystal from reacting with each other.

Thereafter, an opposite substrate 830 provided with a color filter 832and a second alignment film 831 is provided opposite to the activematrix substrate, and the two substrates are attached by the sealingmaterial 834. At this time, the active matrix substrate and the oppositesubstrate 830 are attached with a uniform space therebetween by thespacer 833. Next, the space between the substrates is completely sealedby a sealing material (not shown). Thus, a liquid crystal 846 is sealedbetween the active matrix substrate and the opposite substrate.

Next, one or both of the active matrix substrate and the oppositesubstrate are cut into a desired shape when needed. Further, polarizingplates 835 a and 835 b are provided. Note that a retardation plate maybe provided between the substrate 800 and the polarizing plate 835 a,and between the opposite substrate 830 and the polarizing plate 835 b.Furthermore, a retardation plate may be provided on surfaces of thepolarizing plates 835 a and 835 b, which are opposite to surfaces incontact with the substrate, instead of between the substrate and thepolarizing plate.

Next, a flexible printed circuit (hereinafter referred to as an FPC) 837is connected to the second terminal electrode 838 b provided in anexternal terminal connection region 852 through an anisotropy conductivefilm 836.

A structure of a liquid crystal display module formed in such a manneris described. A pixel region 856 is located at the center of the activematrix substrate. A plurality of pixels are formed in the pixel region856. In FIG. 14A, the gate signal line driver circuits 854 for drivinggate signal lines are provided above and below the pixel region 856. Asource signal line driver circuit 857 for driving source signal lines isprovided in a region between the pixel region 856 and the FPC 837. Thegate signal line driver circuit 854 may be provided on only one side ofthe pixel region 856. An arrangement position of the gate signal linedriver circuit 854 may be selected as appropriate in consideration ofthe substrate size or the like of the liquid crystal display module.Note that the gate signal line driver circuits 854 are preferablyprovided symmetrically with the pixel region 856 therebetweenconsidering operation reliability, efficiency of driving, and the like.Signals are input from the FPC 837 to each driver circuit.

Embodiment 2

A liquid crystal display module according to Embodiment 1 is describedwith reference to FIGS. 19A, 19B, 20A, and 20B. A structure of a pixelportion 930 in each drawing is similar to the structure of the pixelregion 856 in Embodiment 1, and a plurality of pixels are formed overthe substrate 100.

FIG. 19A is a plan view of a liquid crystal display module. FIG. 19B isa circuit diagram of a source driver (also referred to as a sourcesignal line driver circuit) 910. As shown in FIG. 19A, both the sourcedriver 910 and a gate driver (also referred to as a gate signal linedriver circuit) 920 are formed over the substrate 100 same as the pixelportion 930. As shown in FIG. 19B, the source driver 910 includes aplurality of thin film transistors 912 for selecting the source signalline to which a video signal input is transmitted, and a shift register911 for controlling the plurality of thin film transistors 912.

FIG. 20A is a plan view of a liquid crystal display module. FIG. 20B isa circuit diagram of a plurality of analog switch TFTs 940. As shown inFIG. 20A, the liquid crystal display module includes the plurality ofanalog switch TFTs 940 formed over the substrate 100 and an IC 950formed separately from the substrate 100. The IC 950 and the pluralityof analog switch TFTs 940 are electrically connected by an FPC 960, forexample.

The IC 950 is formed using a single crystalline silicon substrate, forexample. The IC 950 controls the plurality of analog switch TFTs 940 andinputs a video signal to the plurality of analog switch TFTs 940. Theplurality of analog switch TFTs 940 for selecting the source signal lineto which a video signal input is transmitted, based on a control signalfrom the IC.

According to the invention, a liquid crystal display device with a wideviewing angle and lower manufacturing cost than a conventional liquidcrystal display device can be provided.

In the invention, since a conductive film is formed over an entiresurface of a substrate, an impurity from the substrate can be preventedfrom being mixed into an active layer. Thus, a semiconductor device withhigh reliability can be obtained.

In the invention, when a semiconductor device including a top-gate thinfilm transistor is formed, a potential of a back gate is stabilized;thus, a semiconductor device with high reliability can be obtained.

Embodiment 3

Examples where the invention is applied to an electronic device aredescribed with reference to FIGS. 21A to 21H. Each electronic device isprovided with any of the display devices or the display modules in theaforementioned embodiment modes and embodiments.

Examples of electronic devices include cameras such as a video cameraand a digital camera, a goggle-type display (a head-mounted display), anavigation system, an audio reproducing device (such as car audiocomponents), a computer, a game machine, a portable information terminal(such as a mobile computer, a mobile phone, a mobile game machine, andan electronic book), an image reproducing device provided with arecording medium (specifically, a device for reproducing a recordingmedium such as a digital versatile disc (DVD) and having a display fordisplaying the reproduced image), and the like. FIGS. 21A to 21H showspecific examples of these electronic devices.

FIG. 21A shows a monitor of a television receiving device or a personalcomputer, which includes a housing 2001, a support base 2002, a displayportion 2003, speaker portions 2004, a video input terminal 2005, andthe like. As the display portion 2003, any of the display devices or thedisplay modules in the aforementioned embodiment modes and embodimentsis used. A monitor in the invention has a wide viewing angle and can beformed at low manufacturing cost compared with a conventional monitor.Further, in a display portion of a monitor in the invention, aconductive film is formed over an entire surface of a substrate, so thatan impurity from the substrate can be prevented from being mixed into anactive layer. Thus, a monitor with high reliability can be obtained.Moreover, when a display portion including a top-gate thin filmtransistor is formed in a monitor of the invention, a potential of aback gate is stabilized; thus, a monitor with high reliability can beobtained.

FIG. 21B shows a digital camera. An image receiving portion 2103 isprovided in the front side of a main body 2101. A shutter button 2106 isprovided at the upper portion of the main body 2101. A display portion2102, operation keys 2104, and an external connection port 2105 areprovided at the backside of the main body 2101. As the display portion2102, any of the display devices or the display modules in theaforementioned embodiment modes and embodiments is used. A digitalcamera in the invention has a wide viewing angle and can be formed atlow manufacturing cost compared with a conventional digital camera.Further, in a display portion of a digital camera in the invention, aconductive film is formed over an entire surface of a substrate, so thatan impurity from the substrate can be prevented from being mixed into anactive layer. Thus, a digital camera with high reliability can beobtained. Moreover, when a display portion including a top-gate thinfilm transistor is formed in a digital camera of the invention, apotential of a back gate is stabilized; thus, a digital camera with highreliability can be obtained.

FIG. 21C shows a notebook personal computer. A main body 2201 isprovided with a keyboard 2204, an external connection port 2205, and apointing device 2206. A housing 2202 including a display portion 2203 isattached to the main body 2201. As the display portion 2203, any of thedisplay devices or the display modules in the aforementioned embodimentmodes and embodiments is used. A computer in the invention has a wideviewing angle and can be formed at low manufacturing cost compared witha conventional computer. Further, in a display portion of a computer inthe invention, a conductive film is formed over an entire surface of asubstrate, so that an impurity from the substrate can be prevented frombeing mixed into an active layer. Thus, a computer with high reliabilitycan be obtained. Moreover, when a display portion including a top-gatethin film transistor is formed in a computer of the invention, apotential of a back gate is stabilized; thus, a computer with highreliability can be obtained.

FIG. 21D shows a mobile computer, which includes a main body 2301, adisplay portion 2302, a switch 2303, operation keys 2304, an infraredport 2305, and the like. An active matrix display device is provided inthe display portion 2302. As the display portion 2302, any of thedisplay devices or the display modules in the aforementioned embodimentmodes and embodiments is used. A computer in the invention has a wideviewing angle and can be formed at low manufacturing cost compared witha conventional computer. Further, in a display portion of a computer inthe invention, a conductive film is formed over an entire surface of asubstrate, so that an impurity from the substrate can be prevented frombeing mixed into an active layer. Thus, a computer with high reliabilitycan be obtained. Moreover, when a display portion including a top-gatethin film transistor is formed in a computer of the invention, apotential of a back gate is stabilized; thus, a computer with highreliability can be obtained.

FIG. 21E shows an image reproducing device. A main body 2401 is providedwith a display portion B 2404, a recording medium reading portion 2405,and an operation key 2406. A housing 2402 including a speaker portion2407 and a display portion A 2403 is attached to the main body 2401. Aseach of the display portion A 2403 and the display portion B 2404, anyof the display devices or the display modules in the aforementionedembodiment modes and embodiments is used. An image reproducing device inthe invention has a wide viewing angle and can be formed at lowmanufacturing cost compared with a conventional image reproducingdevice. Further, in a display portion of an image reproducing device inthe invention, a conductive film is formed over an entire surface of asubstrate, so that an impurity from the substrate can be prevented frombeing mixed into an active layer. Thus, an image reproducing device withhigh reliability can be obtained. Moreover, when a display portionincluding a top-gate thin film transistor is formed in an imagereproducing device of the invention, a potential of a back gate isstabilized; thus, an image reproducing device with high reliability canbe obtained.

FIG. 21F shows an electronic book. A main body 2501 is provided with anoperation key 2503. A plurality of display portions 2502 are attached tothe main body 2501. As the display portions 2502, any of the displaydevices or the display modules in the aforementioned embodiment modesand embodiments is used. An electronic book in the invention has a wideviewing angle and can be formed at low manufacturing cost compared witha conventional electronic book. Further, in a display portion of anelectronic book in the invention, a conductive film is formed over anentire surface of a substrate, so that an impurity from the substratecan be prevented from being mixed into an active layer. Thus, anelectronic book with high reliability can be obtained. Moreover, when adisplay portion including a top-gate thin film transistor is formed inan electronic book of the invention, a potential of a back gate isstabilized; thus, an electronic book with high reliability can beobtained.

FIG. 21G shows a video camera. A main body 2601 is provided with anexternal connection port 2604, a remote control receiving portion 2605,an image receiving portion 2606, a battery 2607, an audio input portion2608, operation keys 2609, and an eyepiece portion 2610. A housing 2603including a display portion 2602 is attached to the main body 2601. Asthe display portions 2602, any of the display devices or the displaymodules in the aforementioned embodiment modes and embodiments is used.A video camera in the invention has a wide viewing angle and can beformed at low manufacturing cost compared with a conventional videocamera. Further, in a display portion of a video camera in theinvention, a conductive film is formed over an entire surface of asubstrate, so that an impurity from the substrate can be prevented frombeing mixed into an active layer. Thus, a video camera with highreliability can be obtained. Moreover, when a display portion includinga top-gate thin film transistor is formed in a video camera of theinvention, a potential of a back gate is stabilized; thus, a videocamera with high reliability can be obtained.

FIG. 21H shows a mobile phone, which includes a main body 2701, ahousing 2702, a display portion 2703, an audio input portion 2704, anaudio output portion 2705, an operation key 2706, an external connectionport 2707, an antenna 2708, and the like. As the display portions 2703,any of the display devices or the display modules in the aforementionedembodiment modes and embodiments is used. A mobile phone in theinvention has a wide viewing angle and can be formed at lowmanufacturing cost compared with a conventional mobile phone. Further,in a display portion of a mobile phone in the invention, a conductivefilm is formed over an entire surface of a substrate, so that animpurity from the substrate can be prevented from being mixed into anactive layer. Thus, a mobile phone with high reliability can beobtained. Moreover, when a display portion including a top-gate thinfilm transistor is formed in a mobile phone of the invention, apotential of a back gate is stabilized; thus, a mobile phone with highreliability can be obtained.

This application is based on Japanese Patent Application serial No.2006-297009 filed in Japan Patent Office on Oct. 31, 2006, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A liquid crystal display device comprising: asubstrate; a transistor comprising: a gate wiring provided along a firstdirection; and a semiconductor layer over the gate wiring; a firstelectrode and a second electrode electrically connected to thesemiconductor layer; a first insulating film over the transistor; apixel electrode on and in contact with the first insulating film and thefirst electrode; a common electrode over the substrate; a wiringprovided over the common electrode and along the first direction; anelectrode on and in contact with the first insulating film and thewiring; and a liquid crystal over the pixel electrode and the commonelectrode, wherein the electrode is in contact with the wiring in aregion overlapping with the common electrode.
 3. The liquid crystaldisplay device according to claim 2, wherein the semiconductor layercomprises amorphous silicon.
 4. The liquid crystal display deviceaccording to claim 2, wherein the pixel electrode has alight-transmitting property.
 5. The liquid crystal display deviceaccording to claim 2, wherein the common electrode has alight-transmitting property.
 6. The liquid crystal display deviceaccording to claim 2, wherein an end portion of the semiconductor layeroverlaps with any of the first electrode and the second electrode. 7.The liquid crystal display device according to claim 2, an orientationof the liquid crystal is controlled by an electric field generatedbetween the pixel electrode and the common electrode.
 8. The liquidcrystal display device according to claim 2, wherein the wiringcomprises the same material as the gate wiring.
 9. The liquid crystaldisplay device according to claim 2, wherein the electrode comprises thesame material as the pixel electrode.
 10. A liquid crystal displaydevice comprising: a substrate; a transistor comprising: a gate wiringprovided along a first direction; a semiconductor layer over the gatewiring; a first electrode and a second electrode electrically connectedto the semiconductor layer; a first insulating film over the transistor;a pixel electrode on and in contact with the first insulating film andthe first electrode; a common electrode over the substrate; a secondinsulating film over the common electrode; a wiring provided over thecommon electrode and along the first direction; an electrode on and incontact with the first insulating film and the wiring; and a liquidcrystal over the pixel electrode and the common electrode, wherein thegate wiring and the wiring are on and in contact with the secondinsulating film, and wherein the electrode is in contact with the wiringin a region overlapping with the common electrode.
 11. The liquidcrystal display device according to claim 10, wherein the semiconductorlayer comprises amorphous silicon.
 12. The liquid crystal display deviceaccording to claim 10, wherein the pixel electrode has alight-transmitting property.
 13. The liquid crystal display deviceaccording to claim 10, wherein the common electrode has alight-transmitting property.
 14. The liquid crystal display deviceaccording to claim 10, wherein an end portion of the semiconductor layeroverlaps with any of the first electrode and the second electrode. 15.The liquid crystal display device according to claim 10, an orientationof the liquid crystal is controlled by an electric field generatedbetween the pixel electrode and the common electrode.
 16. The liquidcrystal display device according to claim 10, wherein the wiringcomprises the same material as the gate wiring.
 17. The liquid crystaldisplay device according to claim 10, wherein the electrode comprisesthe same material as the pixel electrode.
 18. A liquid crystal displaydevice comprising: a substrate; a transistor comprising: a gate wiringprovided along a first direction; and a semiconductor layer over thegate wiring; a first electrode and a second electrode electricallyconnected to the semiconductor layer; a first insulating film over thetransistor; a pixel electrode on and in contact with the firstinsulating film and the first electrode; a common electrode over thesubstrate; a wiring provided over the common electrode and along thefirst direction; an electrode on and in contact with the firstinsulating film and the wiring; and a liquid crystal over the pixelelectrode and the common electrode, wherein the common electrode isprovided over an entire surface of the substrate, and wherein theelectrode is in contact with the wiring in a region overlapping with thecommon electrode.
 19. The liquid crystal display device according toclaim 18, wherein the semiconductor layer comprises amorphous silicon.20. The liquid crystal display device according to claim 18, wherein thepixel electrode has a light-transmitting property.
 21. The liquidcrystal display device according to claim 18, wherein the commonelectrode has a light-transmitting property.
 22. The liquid crystaldisplay device according to claim 18, wherein an end portion of thesemiconductor layer overlaps with any of the first electrode and thesecond electrode.
 23. The liquid crystal display device according toclaim 18, an orientation of the liquid crystal is controlled by anelectric field generated between the pixel electrode and the commonelectrode.
 24. The liquid crystal display device according to claim 18,wherein the wiring comprises the same material as the gate wiring. 25.The liquid crystal display device according to claim 18, wherein theelectrode comprises the same material as the pixel electrode.